Exactly, you can get sub cables that are "y" s. You connect the y end at the sub's plate amp, one into the right channel, the other in the left. The side of the cable with the single RCA goes to the X2 and into one of the Sub Out RCA females, right for the right side sub or left for left. There is a switch on the back of the X2 to select for mono or stereo subs. You could also use a short Y and a single ended sub cable if you like, I have done it this way also and it works just as well. You can use the X2 with a single (summed) subwoofer signal but when you have actively crossed stereo subs the result is very satisfying. After adjusting the sub positions and the X2 levels you will have really good integration without the boominess that cpaul referred to in his post. A well adjusted sub system should not be obnoxiously loud and boomy. It should, sort of, disappear except for the improved low end extension. Even in a small room, using actively crossed stereo subs is still appropriate because of the greater control that you will have.
Jamie