I'll preface this by saying that I haven' owned either of these. However, that being said, I actually think there is probably room for both in your arsenal if you wanted them. I think that the RST-X2 would be the better choice to go between your X1 and Phaze II, and then the UC2 could fit under your Phaze II.
Even though both the X2 and UC2 are asym pearls, I think that there is a pretty significant strength difference there with the X2 being quite a bit stronger by nature. The UC2, from everything I've seen, is a good ball for transition as you need to start opening up your angles late in the day/block. The X2 seems more like a stronger asym that you go to when your big ball starts reading a bit too early, and you want a similar look without having to totally jump zones.
Again, that's just my take. I have NOT thrown these. However, that goes along with what I've seen of these balls in person and/or on video. Good luck!