This mod adds a second XJT module as an internal secondary transmitter, which allows you to use two D-series receivers (PPM protocol with two D-series receivers = 16 channels with TSSI telemtry)
or you can use a combinations of D-series and X-series receivers also to increase the number of receiver channels
(PXX protocol with four X-series (X8R) 8CH receivers = 32 channels with SWR telemetry).
In this mod, I will use the FrSky External XJT module as the primary transmitter with the Txd and Rxd telemetry wires through the back case.
This allows me to use the XJT as a DJT module, the XJT setting needed to change into a DJT: protocol PPM, sub-protocol: D8, dip switch 1: ON (up) and dip switch 2: OFF (down).
At this point in time we are not able to firmware choose which XJT module to have telemetry priority and do not have access to the internal XJT module's manual Bind button (needed for PPM bind), dip switches and to change internally the telemetry Txd Black and Rxd White wires around between S.Port telemetry (PXX with SWR telemetry) and D-Series telemetry (using PPM with TSSI telemetry).
There may be in the future more hardware mods and erSky9x firmware updates that will improve the 9Xtreme features. XJT Module Pin Connections to 9Xtreme Internal Module Connection Pads:
Module Pin 1: PPM / PXX ................................... Pad 9: PPM-PXX signal
Module Pin 2:
Module Pin 3: IN_PWR ...................................... Pad 7: IN_PWR
Module Pin 4: GND .......................................... Pad 8: GND
Module Pin 5: (XJT / PXX protocol / Txd Telemetry Signal Black wire.)
This is as far as I have gotten so far.
More to come!

See here for more 9Xtreme information: (viewtopic.php?f=95&t=7182).
Bob B.