Probably some setting in your DX18. On the iX12 I had to choose and option to bind at 11ms. Bind them both at the same time they both bind at 11ms.

Bind in 11m if using 10 channels or less. Bind in 22ms if using 11 channels or more.
DSMX is limited to 10 channels when using 11ms. Refer to the following from Spektrum.

There are 7 channel data slots per transmission (phase). In high-speed mode (all DSMX receivers are high-speed except a few real old ones like the AR6110),there are two phases sent 11ms apart

In 11ms mode, the first phase has 4 channels that are always the 11ms channels (AILE, ELEV, RUDD and either THRO (multi rotor mode) or AUX1 (acro, sail,heli modes), and the remaining three are the other channels. There are two phases, so what you have going out is 4 channels at 11ms and 6 channels at 22ms. 4 + 6 = 10.

That’s why you are only ever going to get 10 channels if you’re in 11ms mode.

Thanks everyone for the info. I have not had a chance to try and rebind simultaneously, but will report when I do. Makes sense, I bet since I bound separately it's picking up on the first Rx it sees which would be the sats (using ix12).

If you are using the iX12 there is an option you can choose 11ms. I'll look for it when I get home.

Sorry forgot about this I will try to remember to update.