Which “transmitter” do you want to use for this communication? I take it you don’t want to use a transceiver like the RFD868?
I’m pretty sure only one radio connection is needed to use Mission Planner. I’m not positive, but I believe this radio connection needs a way of sending and receiving Mavlink messages.
This thread was originally about using a normal RC transmitter/receiver pair as a joystick input to Mission Planner. The solution @charles_linquist came up with replicates the work discussed in this thread I linked to earlier. (Or the technique mentioned in the other thread replicates the work of @charles_linquist. I don’t know which method was developed first. Bother techniques were likely independently developed.)