I just acquired a BX500 and used it last night - first gig I've played in over a year and a half. My cabs are a pair of 2x10's (SWR's Workingman's) and this setup turned out to be exactly right for me.
I played with no PA support in a group of older guys - no rock stars turning it up to "awesome" - but two guitars, keys, drums, and a sax (yes, he played through the PA). I ran the master on the head at 4.5-5 pretty much all night, the "Drive" up around 7-8, and everything else rather flat. Actually I had the "honk" of the high-mids cut just a little, but that's about it. No "Contour" scoop and the compressor knob set at around 10 o'clock for the evening.
It's doubtful that I could have made enough sound with only one of my cabs. They're acceptable, but not scary gi-normous sounding individually. I've sampled one or two different neo 2x12 cabs in recent years that seemed entirely capable of making very big sound on their own, but I'm not familiar with your Carvin.
If your cab doesn't do enough on its own, I agree with our pals that the most likely recipe for success is a 2nd BRX 10.2. As long as that thing isn't buried down on the floor though, I suspect it's going to make a decent showing all by itself. If you get shopping a 2nd cab that's a different brand/model, proceed with caution.