If there was an additional setting that would tell poly-chain to randomize or alternate voice allocation between the two synths. If you had the OB-6 and P6 patches reasonably close, the result could be very rich tonally.
This doesn't make sense as a built-in feature, but you could certainly achieve this with the aid of a computer and a simple Max/MSP or PureData patch.
I'm guessing this feature would be a bit too obscure to justify implementing it. But while I'm at it... (and maybe this has already been requested): Could you add a way to limit how many additional voices are added when poly-chaining two P6s? For example I bet many people would love to have the choice between playing an 8-voice, 10-voice or 12-voice P6 or OB-6........ 
Are you asking for split/stack functionality when poly chaining two P6s or OB-6s? While this would be fun, these instruments don't have the UI to support bi/multi-timbral operation, and the "what you see is what you get" interface is important to us in terms of our design philosophy for the P6/OB-6. We aren't likely to add new functionality to these instruments--they are pretty much feature complete at this stage.
If you are asking for the ability to disable voices when poly chaining, I'm not sure what the benefit would be to restricting the number of available voices. The Prophet08/Mopho/Tetra line allow you to select the number of voices used when poly chaining, but this is because the various models have different voice counts.
Since the P6/OB-6 models all have six voices, poly chaining two P6s or two OB-6s will always yield 12 voices.
I hope this helps to clear things up regarding poly chaining.