NRPN (MSB, LSB):
Layer A Volume: 0, 29
Layer B Volume: 16, 29
I haven't used CC, but it's listed in the manual as CC #37. Doesn't say anything about layers though, but #38 isn't listed so perhaps it could be Layer B's volume? Otherwise I suspect you'd have to set the Rev2 to multi and send CC #37 on separate MIDI channels, but that's just a guess. Use NRPN if you can, in case you should want to control other parameters later on.