Clock signal is inherent in any digital stream. You could clock them together via SPDIF. Can you record analog in if the clock is set to SPDIF though, that's the question.
on the HD-P2, the clock source is set independently of the signal source. so, although I have never tried it, I'm pretty sure that you can record via mic-in or line-in, and set the clock to S/PDIF. so that you send an S/PDIF signal from one HD-P2 to the other, and the one you are sending it to only uses it as a clock source and ignores the audio data. pretty clever...