They're looking for suggestions for an Ardupilot PRO. They want to run dual Arduino CPUs on this. One CPU should be running a fail-safe program, and also an XBEE signal. The second can be running the current Ardupilot software. This would let the user create an XBEE based radio and completely ditch the need for an external standard receiver. I personally would figure out how to put TWO Xbee sockets onboard. One would be running 900MHz (40kbps) and carry critical data and the controller information, while the other would be a 2.4GHz data uplink (250kbps) for anything the Ardupilot or other add-on cards needed to send to a PC. This would allow either a dedicated 900Mhz signal for longer ranged manual control and emergency signalling, or a double failsafe for manual control with extra bandwidth to ride on for the standard 2.4GHz modules. Duplicate this down at the radio and you'd be set.
Technically, using something like Ardupilot shields, you could have one shield providing IMU, GPS, and servo support, and another shield providing transmitter buttons, analog sticks, a display, and an FTDI or other chip based USB uplink to the computer.