(I have a Beh. dcx2496, so I can run an extra sub channel from there)
Every single person in this thread missed this line. You've got processing available, you can make it work! I do this on a regular basis, with great results: http://soundforums.net/blogs/evan-kirkendall/22-fun-phase.html
If you don't have the measuring tools available, it just takes a little listening. There's 2 simple ways to do it:
First, put the QSC sub on one send, and the USC1 on another. Put the usual processing in for your USC1, and leave the QSC flat.
1. Place the subs side by side. Play some music through them both. Start adding delay time on the QSC until the low end gets louder. Find a sweet spot that seems to really sound punchy and full. Now add like 3-6ms of delay and see if it gets quieter. If it does, you know you've found the sweet spot, if not, keep trying until you can get it.
2. Invert the polarity on one of the subs. Again, add delay time on the QSC sub, but this time the sweet spot is when all of the low end disappears(or just sounds really strange when you put your head between the 2 subs). Now flip the polarity back to normal- the low end should get a lot louder. If it does, you win. If not, start over and keep trying.
I'd imagine 7ms or so of delay would get you close. Remember- this business is all about experimenting and learning!
(but the easy solution would in fact be to buy another USC1 and sell the QSC sub, but what fun is that?)