No.
Both tx's run the AR9020 rx, but it's the X-Plus module that gives you the back 9.
Your crawler, was the rear steer servo & winch esc plugged into the same ch on the rx?
One of the 3pos switches on the 8, 9 or 18 can be assigned to F Mode (flight mode), and to my understanding that's how the same stick or switch can be used for more than one ch, you can program up to three flight modes... I haven't tried it myself.
I like using the rotary sliders on the DX18 to control the tracks, on an excavator, for example... on the DX8, for some reason using a Sabertooth dual esc with it's own built in mix turned on, I couldn't get the dial for proportional for/rev & a 3pos for steering to work together... I eventually figured it out to incorporate F Mode (manipulating EPA & Reverse ch of the 3pos) to control steering so the dial could be used for for/rev, but what a headache to fine tune that precisely.. and it didn't work all that good anyway.
The dials & sliders are proportional control, like the sticks, where as the switches are on/off/on. You can adjust EPA + - to how much rpm an electric motor has, but it's either on or off, nothing in between.
You could also use the dial or sliders to control a servo... the servo arm attached to something you want to hold in a specific location at any given time indefinitely.