Rooze, I think it depends on the design of your amps, but REL recommends two subs with most bridged mono setups connected from the speaker terminals on the amp to the high level inputs on the sub, one sub on each amp. I'm assuming you have one amp on each speaker now. In that case, no need for a separate active XO and it would not go through the maggie xo either. You would still send the full range signal to the speakers and the subs would be set to fill in the lower frequencies where the mains roll off.
You may need to contact the amp manufacturer to get their recommendations, or someone here may know if you provide more specific information.
You may need to contact the amp manufacturer to get their recommendations, or someone here may know if you provide more specific information.