Interesting product from HobbyKing.

http://www.hobbyking.com/hobbyking/store/__18703__Wireless_Buddy_Box_System_Dual_RX_Controller.html

Wonder if those female to female cables are readily available, or people will end up cutting a lot of servo extensions.

Anwar... how does this work?...

It has 8x electronic 2 way switches (like CD 4066)  , with the EN pin connected to one Aux channel of the Master Rx... As long as the Master Tx has put the Aux channel OFF, the single from Master input goes to output. As the  master puts the Aux channel ON (EN), the single from the Slave side appears on the output.
                         ___________
                        |                 |
                  EN   |                 |
          _________|_________   |         
          _________|_______\ |  |   
            MAS      |             \__|___  SRVO
          _________|_______      |
            SLA       |                  |
                         -------------

                         ___________
                  __   |                 |
                  EN   |                 |
          _________|_________   |         
          _________|_______   |  |   
            MAS      |             __  |___  SRVO
          _________|_______/     |
            SLA       |                  |
                         -------------



CD -4066 Quad analog switches.
   

      +---+--+---+
  1X |1  +--+ 14| VCC
  1Y |2         13| 1EN
  2Y |3         12| 4EN
  2X |4         11| 4X
 2EN |5  406610| 4Y
 3EN |6          9| 3Y
 GND |7         8| 3X
     +----------+


Well my electronics info is  dated and this device might be using Opto-coupler based switch instead of CMOS ones..

Is it that simple with a PWM control signal, as opposed to a plain high/low signal on the EN pin ?

Basically you have two receivers on the aircraft, controlled by two different transmitters. One of the transmitters is designated master, because one on its channels is connected to the "control" channel of this device, and the signal on that channel determines which receivers channel outputs are actually fed into the servos.  So by flicking a switch on the master transmitter, it can decide whether the student radio's receiver signals are fed into the aircraft, or the master's itself.

The good thing is that the two radios can be pretty much anything.  You can mix brands, and bands (2.4Ghz and 72Mhz, for example).  Another good thing is that the student and master radios are not tied up physically by a buddy cable, so they don't have to be that careful about keeping the cord intact.  The bad thing is that both radios have to be individually programmed with all settings for this model.  And you will have to find space for an extra receiver, this device and related wiring.  The additional power drain due to the extra receiver should be negligible.

Manual : http://www.hobbyking.com/hobbyking/store/uploads/1010167278X22035X20.pdf

So this is an Assan product. 

CD 4066 is has 'Analog' switches lines not TTL. I used it long time back in my Homebrew HiFI Audio Deck to control the audio channel between Tape/Aux. It was my assumptions, but similar multiplexer should exist to handle PWM signals... Could anybody currently active in Electronics shed more light on this ? I left active hobby electronics back in class 11th ...we hardly had anything more than 8085 uP back then, which itself was out of reach of most of us (PC used to have 2-8 MB RAM and 650MB HDD !)

I was just thinking aloud about what must be going on inside that device... not really about specific devices, which I am pretty much clueless about ! 

I left the world of electronics soon after I entered it.  I did start programming on a Sinclair ZX Spektrum with 48KB of memory and a Z81 processor while in class 9, with regular audio tapes as storage medium, and the regular TV as the display   BASIC and Assembler were the only language choices.

Oh!! Dont remind of Spectrum Jr, Tape Cartridges, and running the whole tape to fetch a file... I am not even talking about Floppy drives!!!  GWBASIC, QBASIC, DOS , etc came a little later