Sennheiser AVX system in FCPX

Hi all,
I am wondering if anyone else has found the solution to this problem. I just did a shoot using the newish Sennheiser AVX system which has about 19 ms of latency. I think (not sure) that means I need to move the audio about half a frame back to get perfect synch. The problem is that we mixed an overhead boom mic (analog of course) with the signal going into a RED Scarlet-MX. So we have on channel 3, the digital transmitters and on channel 4, the analog signal. When I import the clips into FCPX, they of course get grouped. I detach the audio and then expand the audio components. No problem. However, I cannot figure out how to separate the channels (I have them set to dual mono in the inspector) to allow for slippage of channel 4 so that it will match channel 3. Anyone know how to do this? Also, does anyone know how I might interpolate 19ms to frames and fractions of frames in FCPX?

Thanks