Phase shifts can make a big difference. Without a sub crosover you can shift phase 180 degrees by inverting the polarity at the speaker or amp. Speaker placement is still the most important consideration though.
If your room is symetrical and you can place the subs symetricaly in the room (and still move them about for optimal performance) then stereo subs is the way to go. Most sub controlers are going to shift phase and change levels equaly to both subs.
If your room is not symetrical (or you can't place them symetricaly) you will be better off with one sub placed properly. Plus one sub is about ten times easier to set-up than two.
Good powered subwoofers are easy to set up and sound very good. It helps tremendously to have a patient helper to adjust the controls for you while you sit in the sweet spot.
Good luck!
