Hi, Maurice!
The tranfer lead 85120 was only designed for the transmitters mc3010, mc3030 and mc4000 to tranfer data from one transmitter to the other (mc30XX to mc30XX and mc4000 to mc4000). Although it uses the plug that matches the ROYAL SX there is no use of it: the transfer of data between two ROYAL SXs (evo; pro) via a lead is not possible.
The lead 85120 is also of no use concerning buddy to buddy activities. For this you need the lead 85121 depending on the second transmitter you want to use.
If you want to give assist to someone who has got an M-LINK transmitter you need the buddy-stick 45183 for your ROYAL SX, no matter if you ROYAL works on 2,4 GHz or 40/35 MHz. Then the link is wireless!
For further advice, please state what system you want to work with including GHz/MHz and the exact transmitters!
best regards
profimc