chad ellis is wrong, pimpmyride is right.
150W is not much power; although 2 subs getting 150W each will likely give you decent SPL (especially if they're 12's or larger) BUT you're not going to get much out of the sub in terms of depth/detail/dynamicness etc. It takes a good amount of power to be able to drive/control a sub to make it sound good. In general, 1 300W sub will sound overall better than 2 150W subs.
Pimpmyride's sub recommendations are good.