Hi. I've 2 Sony CX900s and am experiencing the same unacceptable "stacking end-to-end" issue when I try to make a multicam. Each was called "Clip 1." I tried changing that but was unsuccessful in getting FCPX to correctly synch my footage.
FYI - I'm writing this from the Apple Store. I just stumped my 1:1 FCPX guru. :(