I'll do my best to answer, not sure I'm getting this 100% right but I'm sure someone will correct me

The stock receiver for the RX18 is separate but it's a 27mhz receiver or something (either way it's crap)
To use a 2.4GHz radio with the RX18 you'll need something like the RC2HL (
http://rctanksaustralia.com/shop/index. ... cts_id=196 to read about it) which is an adapter board you connect to your RX18 (same slot as existing 27mhz receiver is plugged into)
You then connect the RC2HL to one of your spektrum receivers and you'll be good to go
