There is really nothing to stop two receivers being in the same boat, they just each need their own transmitter. Not a very elegant way of getting 6 control functions, but feasible. Ish.
A model control receiver is not much like a normal radio which just feeds its output to a loudspeaker. It takes an encoded signal which is carrying the information for the various output channels and distributes them, using hardwired logic circuits, to the required output slots. In the case of a 3 channel set, the hardware is designed for 3 channels and that is what the hardware is designed to count up to. Adding more requires a complete rebuild of the output stage, taking another and sort of grafting it on is not an option, since having the ability to count up to six is not the same as counting up to three twice.
