They changed the TPMS sensors sometime in 2010 - completely incompatible before vs. after the switchover.
I just found this out myself... I have a winter setup that I used on my 2009 X6 50i--and replaced that car a few months back with a similarly equipped 2013 X6 50i. Figured I could use the same wheels for winter. TPMS would not reset (i.e., constantly said "TPM is resetting"). Quick googling showed many BMW search results about the change.
So, the good (?) news is you can just order the newer TPMS sensors and corresponding new vavle stems and have them swapped out. I'm having this done as I write (~i.e., car literally at dealer right today). It is $113 per sensor plus extra for install, etc. Not cheap. Tirerack can sell you all 4 sensors and stems for $200 total it seems like from some research... I just didn't have the time to order and find a shop to install--and was more convenient for me to have the dealer do this since my local one will do a loaner car, etc.
As for your X5 wheels, I'm not so sure they will directly go on the X6... there might be a different offset. Not too knowledgable on that aspect but figured would share the TPMS story ;-)