If you need to be hitting a particular post mash wort volume for your boil, you should be able to back calculate how much grain you can use. If you are shooting for 3.6, you would be using slightly more (since your grain will soak some up). Assuming a batch sparge (which is essentially what BCS's two pot method is), if you use 4 gal total (I think that some grains will likely retain more water than others, someone else may have a better idea on this), you could use 2.25 for mash and 1.75 for sparge (others feel free to chime in with better numbers). Using 2.25gal for mash will give you 9 quarts. If you assume a grain to water ratio of 1:1.5 (again, can vary), that will mean you would use 6lbs of grain. Out of those six pounds, subtract out your specialty grains and make up the balance with base male.
You also need to take into account the size of your available pots... if your second pot is small, you might not have room for that much grain and would be better off doing a smaller partial mash and adding water to bring you up to your 3.6gal volume. This is the case for me, I've been using the bottling bucket as one of my pots, adding my strike water to that and then doing my sparge by dunking the bag in the brew pot.
PFC, Arachnid Deployment Division
In the cellar: Galaxy Pale, Citra Pale, Honey Common
In the fermentor: Cat Yakk Saison
In the works: Wooden Cider