Per the instruction manual, wireless copy is between two TTX850's. The TTX650 does not have that capability.
That is strange, I have both the 650 and 850 and I use the wireless buddy box both ways; i.e. with either as master with the other slave. Works perfectly. I believe the 640 also works with either the 650 or 850 as a slave. Have not tried that as I do not have the 640.