I don't think exchanging sources will help much. True, they were developed from the same base, but I think that the changes made in the kernel area were pretty different due to 32bitting one, due to different hardware platforms and due to different intentions. Thus I'd expect the source trees to be pretty different by now so that it would be hard to take changes from one side over to the other one.
I rather expect that we'll have two base OSses for some time, the 26 bit RISC OS 4 thread and the 32 bit RISC OS 5 one. The things sitting on top of the base OS - be it Select features or Merlin - will probably (hopefully) become pretty much the same as time comes by, as will surely the OS interfaces, Toolbox, ... Having a modular OS makes these things feasible, I'd say.