It works here, although it does seem to be a bit finicky. A couple of things..
- Are you able select the different mics for different channels [Smart Controls pane > Track > Input submenu]? If not, you might have to create an Aggregate Device in the Audio MIDI Setup utility.
- If you *are* able to assign each mic to its own track, then you'll have to click the Record button on both tracks, so you can see incoming level on the meters for both tracks and they're both lit up & flashing when you hit Record -- it sounds like that's what you're doing. That's where I found GB to be finicky -- sometimes it wouldn't let me arm both tracks, and other times when I did have them both armed the unselected track recorded and the other track dropped out of record mode when I hit Record. After a couple of unsuccessful tries, then -- for no apparent reason -- it started working properly, and after that it always worked. I don't know know why it was dodgy at first, but as long as both tracks show input and are record-ready [flashing] they both *should* record [regardless of which one is actually selected].
If it's acting up you can always try a hard shutdown/restart -- that sometimes fixes those mysterious "I'm doing everything right but it's not working" problems..