MacBook Pro (Retina, 13-inch, Early 2015) Dual External Monitor Performance
I had this exact same question, and found the answer on Apple's discussion forums:


  A final update - I got my hands on two Dell P2715Q's which support 3840x2160@60Hz over SST. I'm happy to report the new 13" RMBP happily drives both displays at their native resolutions with 60Hz refresh in single stream mode (the default).


https://discussions.apple.com/thread/6868583