Have 2 sub woofers curently in front stage. Should I put one in back and one in front?

Depends on your room. If they fit nicely in the front, I'd leave them there unless you have a particular issue to solve.

I have 1 Tower up front but the back of my HT is open to the stairs going up. In that situation, would adding another sub in the back be better than adding another sub to the front? If I do add a sub to the back, would I need to adjust the phase on the one in the back? The phase can either be 0 or 180. I do notice some loss of bass in the back row of seats which is about 4ft from the stairs.

What about cancellation issues as well?

Try placing each sub close to each of the front towers and see how much value it brings to 2 channel stereo music listening.

I plan on getting two subs just for that.
It should also provide more balance when watching movies.

They should be up front below the centre channel.

If you have a standard shoe-box dimensioned rectangular room and care about bass uniformity across several listening chairs, one front center, one rear center placed subwoofer seems to be recommended (Harmon). If you want more bass, collocation will grant you the ~ +6dB gain.