If I understand you, you have a single master now that feeds all four drum brakes. You want to install a master with two chambers, with the front chamber feeding the rear brakes and the rear chamber feeding the front brakes. You want to know what size bore to use in the master cylinder to get the same pedal feel, travel and displacement of fluid. Does that about sum it up?
If so, you will have to determine the bore of the master that's on the car now. Let's say that it's 1 1/4". I have no clue what it is, but we have to start somewhere. If you find the square inch area of that bore by multiplying .7854 X bore X bore, you find that it is 1.227 square inches. So, lets say that with that bore, when you press on the brake pedal, the piston in the master goes 1 1/4". Again, I don't have a clue what it is, I'm just trying to show you how to figure this out. If the bore is 1 1/4" and the stroke is 1 1/4", then the volume displaced on a brake stroke is 25.13 cc's. (.7854 X 1.25 X 1.25 X 1.25 X 16.387 = 25.13) Now, if you are going to split the master and have two separate circuits, then you will need to cut the displacement in half on each circuit to displace the same total amount of fluid. Let's say that you found a master with a 7/8" bore. With a 1 1/4" stroke, each circuit would displace 12.3 cc's, so two circuits would displace 24.6 cc's. (.7854 X .875 X .875 X 1.25 X 16.387 X 2 = 24.6) That's pretty close to the 25.13 cc's you were getting with the single master and probably would work ok.