I can't find any documentation about using the new cellular models built-in LTE in addition to a USB dongle. I'm going to assume you can't. You'll just have to try it out lol.
What I do know will work (even though its not technically supported) is using two MX67C. Say one has Verizon and the other AT&T. So you have your redundancy carrier wise. And you can setup warm-spare with them (again not officially supported), but it does work. Fail-over won't be seamless, its about 5 seconds or more fail-over from my testing.
If your looking for more flexibility I think you could get two cradlepoints as your LTE 'modems', and plug them into WAN 1 and WAN 2 and the MX won't know they are LTE devices and just act like they are normal circuits. That would also allow you to mount them wherever you want via Ethernet cable, so they won't be stuck where the MX has to sit the way the built-in modem in the MX has to. Might get better signal that way etc.