There's some members using at least two subs in the HT (including me) :yes:
What you need to consider when adding an additional sub ... is to use the same subs if possible (or at least with the same specifications); or the sub with the worse specifications won't help the other :yes:
The low frequencies from the surrounds and rears should already be routed to your single sub. The frequencies should also be non-directional (assuming an 80hz crossover), so you don't need a second sub in the rear. If you do go with2 subs, go for the placement that gives you the best performance, regardless of whether that is at the front or back of the room.