If you deinterlace the stereo stream into two independent left and right streams on the source pi then snapcast can’t sync between them. I also didn’t think it is currently possible for a snapcast client to request just one channel, but I might be wrong there since you can imagine that being a good feature for exactly this sort of setup. Either way, if you want to get something working as a proof of concept then I would start with simply muting the channel you don’t want at each sink pi.