I thought that Mike Maxwell had said that "time-slicing" using one radio for both Zigbee and Thread wasn't an approach he liked at all when we were discussing Thread support some time back. I was thinking/hoping that meant whe HE took on Thread it would have a separate radio for it. I haven't been keeping up on that part of what we see in the FCC stuff so don't know if there are clues there about one or two radios to support Thread and Zigbee.
Ah - just found this from @rlithgow1 on radios in the "FCC C-8" thread. I would assume that if we get two radios no time-slicing required. 
