This thread on RC forums they worked out how to do it.
http://www.rcgroups.com/forums/showthread.php?t=2622713
“The secret is to bind both receiver at the same time.”
“Once the simultaneous binding is done, we must remove the binding cable BEFORE turning off the the receivers!”
“By binding the receivers separately, I always ended up having only the last one bound. The first one would always unbind.”