I checked on my REV2 in mono mode and to listen to layer B only, I simply press Edit Layer B button and I can hear layed B. If that button isn't lit up, I hear layer A only. Of course, pressing Stack A+B will hear both layers together, and pressing Split A/B will play either layer A below a certain note, and layer B higher than that note played on the keyboard.
You could always check with headphones plugged in the Phones output to make sure that everything is in mono there ?
Hope this helps ?