I have an XL2 and 1 sennhizer wireless lav mic.

Normally I have this plugged into Cahnnel 3 and the monitoring button set to channel 3/4

If I plug a second sen wireless lav mic into channel 4, how can I monitor each lav mic without hearing the audio from the other lav mic? or can you?

Well, you'd have to turn down ch 3 and ch 4 gain independently but that won't work while recording, only for presetting levels. There is no way to monitor from a single channel only. You can't do that with ch 1 and 2 either.

-gb-