Hmm offhand I don't think you can get two leafs to fire simultaneously with a dual cable release. I doubt the timing will be anywhere near that precise, and you probably couldn't even use a tunable delay to synch them because the cable probably won't give repeatable results. What you need is electronic control, not mechanical. E.g. you could use the x-synch output from one lens to trigger the other, or just send one timing pulse to both. But even in this case, I have doubts that the leafs will synch perfectly unless the shutters are in tiptop condition.
How about using a focal plane shutter system e.g. a speed graphic. That'd be simple enough. You can put a rollfilm back on that, no problem. I don't know offhand if the planarity would be good enough but maybe.
Mind you, if you don't need speeds faster than 1/25 or so, you could just use a packard shutter. But then again, for speeds that slow, a dual release might be good enough. In 10 mins with a piece of tape and some plastic you could make yourself a plunger to fire 'em both. Probably good enough for longish exposures.
P.S. I have the rb/rz dual releases and they won't work for your application: the two cables are not designed to be synched together; rather, one always goes first.