For sync to work, the CoreXL config must match between the two nodes. That's the only real, functional concern, though. I've done this dozens of times.
Each system should ideally have at least as many interfaces as you are actually using. It's possible to define an interface only on one member, if you're willing to accept that traffic not working when you run on the other member.
I haven't personally tried clustering between an untagged interface on one firewall and a tagged interface on another, but I don't know of a reason it definitely would not work. That might let you get around the interface count concern above.
NOTE: there is a difference between "will work" and "is supported". Doing weird things like this will probably work, but the TAC may not be able to help with problems. As a result, I would not run a cluster like this for long.