Correct cylinder to barrel volume ratios depend a lot on spring rate and bb mass and there is no clearly defined correct cylinder volume for any given barrel length. You can match your cylinder to barrel ratio so as to maximize your fps with .2s but since heavier bbs cause the air in the cylinder to compress more, your fps will not be optimized for heavier bbs. Ideally you would want to optimize your setup so as to get the maximum fps from the bbs that you intend to use, which are usually heavier than .2. Figuring out what that optimal cylinder size is would require you to start off with a full cylinder and then incrementally add a longer port with a dremel and keep chronoing every couple milimeters until you hit your max fps. That would be the point where the cylinder is perfectly balanced for your barrel length.
In your case, and from a practical standpoint I think that in most cases one should just use good judgement and avoid overvoluming if possible to eliminate turbulence due to the extra air. In your case a standard 3/4 M4 cylinder will probably be just fine for both barrels.