A Y adapter will suffice, you have to set up the subs so that they are both calibrated to the same level or slightly lower than the speakers, as the dual subs have more over all output, about 3db for subs in different places within the room and if co-located about 6db more. Once you have them calibated, you can run Audyysey and it should help smooth things out even more.
Having a receiver that has dual sub outputs AND is capable of adjusting each subs output independently is obviously more advantagous as the receiver will make the required adjustments for you on each sub, but not all receivers have this ability yet.
