What are the downsides of parti-gyle brewing?
While this technique is obviously not for the extract brewer, it is also not for the inexperienced all-grain brewer, either. For a start you have to be well-organized up front as you may need extra collecting vessels, and allow for space in which to keep them where you won’t knock them over when they are full of precious wort. You are going to have to conduct two separate boils, which can make for a long brew day if they both have to be done in the same vessel. It would be ideal to use two boilers (and, of course, two heating systems), so it would be good if you could borrow one from a friend. Even better, borrow the friend as well to make the whole thing less work. Please do not think you can make this procedure simpler by keeping the second wort overnight and boiling it the next day. Maybe you don’t believe me because you have done this before and all was fine. Well, I must tell you that you might have gotten away with it that time, but you almost certainly won’t the next time! I’ve tried this and found the next day that all I had was a bucket of foul-smelling acidic liquid fit only for throwing away.
   
You’ll also obviously need to have two clean fermenters ready to go, and enough yeast to pitch both of them. I don’t think you can parti-gyle unless you have at least a 1 qt (1 L) active yeast starter prepared in advance ?D remember one of these beers is big, and needs plenty of yeast to get the required attenuation.
   
Another reason why this technique is for the experienced brewer is that you are going to have to think on your feet and to use some mathematics. You have to think very clearly as to what two beers you are going to make, particularly what starting gravities you want to hit, and the level of IBUs you wish to achieve. You then need to calculate an appropriate grain bill for the mash, as well as suitable hop varieties and addition rates. The latter will come from style guidelines and your own taste, and the formula can be arrived at by the “total gravity points” concept. An experienced all-grain brewer should probably be aware of this type of calculation, and if you are not, you should be, as should any brewer looking to achieve consistency.

Working it out
For those not familiar with the “total gravity points” concept, or how it might work in the parti-gyle concept, let’s look at an example. Let’s say you want to brew 2 gallons (7.6 L) of a barleywine at OG 1.080, and 4 gallons (15 L) of a session bitter ale at 1.040. Then, taking only the significant numbers after the decimal point, we get:

Barleywine points = 2 x 80 = 160
Bitter ale points   = 4 x 40 = 160
Total points      = 160 x 2 = 320

Now, for the sake of simplicity, I am going to suppose that I use only pale malt for this beer and that as is normal with BYO recipes, which assume 65% extract efficiency, 1 lb. of pale malt in 1 gallon of water yields a wort of SG 1.024.
   
Total malt required = 320 ÷ 24 = 13 lb.
So 13 lbs. (5.9 kg) of pale malt would be our grain bill for these two beers. Obviously, it is a simple adjustment if your extract efficiency is greater or less than 65%. It doesn’t look quite so simple if you are going to include malts giving less extract than pale, such as crystal and roasted malts, but there’s an easy way around that. Say you want to brew a dark barleywine/strong stout at OG 1.080, and a “drinking” dry stout at OG 1.040. Just decide up front how much roast malt you want to add; let’s say we’ll add one pound of black malt or roasted barley. This at our 65% extract efficiency will yield a wort of SG 1.016 in one gallon of water. So:
   
Points from roast malt = 16
Points required from
    pale malt = 320 - 16 = 304
Total pale malt
    required = 304 ÷24 = 12.7 lb

But you will probably not know exactly how much wort you are going to obtain, especially if your grain bill is a greater amount than you would normally have in your mash tun. So you must measure the gravity of the collected first wort right away, taking care to cool the sample to the appropriate temperature (usually 60 °F, or 15 °C). Then, using the “total gravity points” concept, you can calculate whether you need to adjust the volume up (by dilution) or down (by evaporation) to get your target gravity. For example, let’s say you wanted 3 gallons (11.3 L) of wort at a starting gravity of 1.080, and you collected 2 gallons (7.6 L) at 1.100 SG. Then, ignoring the figure before the decimal point, you have 2 x 100 = 200 total gravity points. At target gravity of 1.080 you would then have 200 ÷ 80 = 2.5 gallons (9.5 L), so you need to dilute the wort down to this figure with water. Fine, that’s simple enough, but the problem is you (or your brewing program) calculated the hops based on 3 gallons (11.3 L) of wort. So now you have to re-calculate hopping rates based on the new volume. Easy enough, just multiply the original weight by 2.5/3. But do note that if you decided to go the traditional way and to stay with the 2 gallons (7.6 L) at SG 1.100, that hop adjustments are a little more complicated. If you simply multiply the original weight you calculated by 2⁄3 the beer will likely be underhopped. That’s because it will be higher in alcohol and rather sweeter than the beer you had intended to brew and hop utilization will probably be slightly less. You should therefore expect to add 10?C20% more bittering hops, according to your own taste requirements. The second mash can be a lot of fun to play with. First of all, you do not have to do an actual mash at this stage. You can start sparging in the usual manner while collecting the first wort then simply continue to do so while you collect the second wort. But doing a second mash permits you to tinker by adding more malt. In the second wort, for example a dry stout, you might add a little chocolate malt for instance. In such a case you can simply add the extra malt, then hot water at about 170 °F (77 °C), and mix the mash thoroughly then allowing it to stand for 10?C15 minutes to allow formation of the grain bed. The temperature of the water is not critical as you are really leaching out extract, rather than doing a real mash. Do not add too much water so that you leach out undesirable materials, such as tannins. Just add what would be your normal sparge volume in a regular brew.
    
Or, if you got more extract in the first wort than you had expected, so your second wort is not going to hit target gravity, you may wish to add a couple of extra pounds of pale malt at this stage. If you do so the mash water should be at 160?C165 °F (71?C74 °C), and you should let the mash rest for 30?C45 minutes before run off. A simpler approach would be to skip this, do the mash and adjust the wort gravity by addition of a little malt extract.
   
Again, with the second wort you may decide to take it just as it comes and make no adjustments to volume and gravity. In which case, do remember to allow for this in your bittering hop additions. If you don’t, you may well end up with an unbalanced beer, which would be a disappointment after all that hard work!

Doing it for real
If you are confused by the above, here’s what I got when I tried a brew using this technique. I was re-creating a recipe from the 18th century for Dublin Stout; I didn’t want a lot so I calculated that 12 lb. (5.5 kg) of mild ale malt and 0.5 lb. (0.225 kg) of black malt would give me 2 gallons (7.6 L) of this stout at OG 1.090, with a second mash yielding about 4 gallons (15.1 L) at OG 1.040 (using a value for extract efficiency determined in my own brewery). The big stout I estimated from the original recipe should have around 90 IBU, using UK Target pellets at 11.5% alpha-acid. I figured that the smaller beer would be a dry stout, and would have around 40 IBU. I would mash as per usual, and sparge as soon as I ran off wort.
   
In fact, my actual numbers differed from the expected values. What I got for my first parti-gyle was 2 gallons (7.6 L) at OG 1.085, and I wondered if I should adjust this and the hopping rate. I didn’t want to concentrate it, because 2 gallons (7.6 L) was as low as I wanted to go, so I decided to leave the gravity at this figure, and to do the same with the hop rate, as I was only guessing at IBUs in the original recipe. So I went ahead and boiled, hopped, and fermented this beer.
   
The second parti-gyle was about 3.75 gallons (14.2 L) at an OG of 1.038, both values a little lower than I had hoped. Also, the beer was paler than I expected and tasting the wort suggested there was not enough roast character for a dry stout. If I had done two mashes, instead of sparging right away, I could have added some extra roast malt. Or I might have brought the gravity up and deepened the color by adding malt extract, but I didn’t have any such extract on hand that day. So I decided it had enough color to make a “pale” brown ale, and adjusted hop levels downwards to an estimated 28 IBU, then boiled and fermented the beer as normal. All in all it was a good and satisfying experiment with an interesting result, and two beers for the price of one.
 

(from:byo.com)