I was working on that too. But I want to put the rendering of one plugin on the available cores, not different plugins on different cores.
I have a dualcore, able to do 2 threads at once, I put the rendering of half of the voices in a new class derived from Thread and let the other half of the voices in the main thread. The OS (here win xp) is moving the new Thread automatically to the other CPU and the rendering-time is going down to the half (before 60 voices 70%, with the second Thread 60 voices about 37%, with switched off virus-prg and switched off Task-Manager). But I do have some problems with crackling. Right now I haven’t had the time to figure out if there is a mistake in my implementation (waitableevent,criticalsection,or elsewhere) or if the solution itself will not work. Also it could be that you should only use one thread less than you have cpu’s, so that one cpu can always be occupied from the OS or other programs and you don’t have to wait until it is free.