OK this is tricky to get perfect but once it is done its worth it...
First I would tackle crossing, what are you using to cross the mains to the subs? Mini DSP works very well for around $100...
Cross your mains as low as you can so if you like how your mains sound at 50 cross them there, {checking there specs to see where they fall off can be helpful}.
Next placement, I would start with where they look good and dont interfere with your mains placement which is much more important...
Then balance them, using a db meter play a test tone and balance 1 at a time to a certain level, then play them both and record their level together, then play your mains with no subs and put them at the same level, fine tune to your tastes after that...
I would run them both summed, so you have left and right going to each sub not a left sub and a right sub...
Good luck, you will never not use subs again, lol...