This is not gospel but I believe to be correct. I was told by a BMW parts guy that the hubs on an X6 are different sizes... with one (front or rear) being smaller than the X5 so that either the front or rear wheel of the X6 will be to small to fit over the e70 hub. He stated they tried to mount a 20" OEM X6 wheel on an e70 & no worky. I spoke to another BMW guy at one point way back that said they would fit however I trust the "no worky" guy more.
I actually have both e70 & e71 wheels yet have not invested the time or muscle to lug them around to check for myself.