Yes, that's my understanding too Nathan.
The DX18 (for example) uses an AR9020 receiver for the 1st 9chs, then the X Plus module plugs into the AR9020 to give the back 9chs. So if other remotely placed rx's were used, they would be the AR8000 and/or whatever the 6ch rx number is, and whatever rx is using throttle ch, no other rx could too.
There's DSM2 receivers & DSMX receivers. All receivers in series have to be
2 or
X to function together.