If you use moss as a substrate, it can easily lead to belly rot if it's really wet all the time. Probably not the best choice if you ask me.

And yeah...you just got a burm(which you seemed to rush into) and now you're thinking about getting another snake. Geez, give it a month or something. I dunno...you seem to be rushing into your snakes pretty fast. Seems like they're impulse buys...

Also, just because you already have bp's, doesn't mean you should shy away from getting one for your gf. It'd be easier to maintain if it's her first snake. Don't just rush out and buy a hog island boa. Slow down a little and take your time. Consider your girlfriend too. If she's going to be taking care of the snake, then ask her what she'd like. Maybe a ball python or corn snake would be better than a boa.