I've used these:

[link=http://www.hobbyking.com/hobbyking/store/__30896__Wireless_Buddy_Box_System_8CH_Dual_RX_Con troller_US_Warehouse_.html]8 channel Buddy system[/link]

They are receiver agnostic so you can hook up a JR RX and a Futaba RX and use corresponding TX's for masters and slaves.

The only downside is that you have to trim out both the master and slave and of course mixes are handled by each TX.

e.g. You cannot use JR/Spektrum's "Pilot Link" that lets the master's trims apply to the slave.