Inline Tube dual master cylinder conversion

A drum/drum block is just a distribution block so all you have to do is keep the two sides separate, and isolated.This block should have 5 ports on it. If there are only 3 ports on it, then this is a front-only distribution block, and you plumb it to the firewall reservoir... or to the larger one, as the case may be. Then you can run the other M/C port directly to the rear flex-hose. These ports should both have a tiny hold-back valve in each port, but if you have modern w/cs with the metal expansion cups in them, then the hold-backs are optional.
If you have disc-brakes, this will require a DIFFERENT block, which has a proportioning valve built into it.
In this case; there should be two lines from the M/C to the block, plus two lines to the front discs, but only one line out to the rear (which should have a hold- back valve in the M/C port. total 5 ports.
If you have disc brakes, but only a 3-port block, you can buy a stand-alone Proportioning valve to install in the rear line, but you lose the light switch.. If you do not have a dash brake-lite. then you don't need a light switch in the Combination valve.
IMO losing the light switch is not that big a deal because it only comes on AFTER one of your two brake systems has failed, and trust me when I say that you will know the instant this happens, lol, and the light only tells you what you already know.