I don't believe the "X" specifically means a merger of any kind. Canon, in their videos, seems to be referring to the 1D X as their "tenth generation" professional camera. I'm not exactly sure how they get "tenth" generation...counting both the 1D and 1Ds, I can only account for seven separate camera releases myself (maybe eight as there might have been some intermediary 1D II/III N in there somewhere...not sure if that was a legit "generation" or not)...but I could be missing something.
Canon EOS 1v, released in 2000, was the last Canon's professional film camera, where "v" represented 5th generation. Four generations of 1D series and you get 1D X as the 10th generation