Two subs in a small room....

I have read that two subs smooth out the bottom end and give a much better sound. But, what if you have a room that is just not big enough for two subs. Is it worth the effort to get two "small" subs or is there a special placement for just one sub besides the usually place, behind one tower. What about placing the sub right in the middle?

By the way, my system is in a 10 X 10 room and it is in a diagonal configuration if you can picture that.