Hi Phil - Why double belts? a 25mm belt is a strong stiff belt. Plasmas have no contact loads so a 25mm belt is more than adequate? Plus one will be slightly different to the other and they will conflict in some way.
Add up your cost for the V bearing set up, you may find round rails are cheaper and more reliable once you add in all the bits to get a V bearing system working....I usually model a belt without the teeth so its simpler and initially in two overlapping (like two U's) halves. In this way when you change the centres the "belt" changes with it. Once you nail down the design then you can convert to a "single belt" if you wish... what CAD are you using? cheers Peter