is there really a need for two subs?

I agree with some of the comments above. There is a level you have to reach first before two is better than. Generally, one $1500 sub will out perform two $750 subs. Two really good subs will be even better. The sub does not have to do double duty with the bass signal from 2 channels.

I don't agree stereo subs are harder to blend. They are each playing separate stereo channels. No different than two speakers (just the lows). If you have problems with stereo subs, you will have the same problem with the bass response of your two main speakers. It is not the subs.

Now using double subs (two subs playing the same signal) might pose a blend problem.