Absolutely. There are a few ways to do this.
You can add the interface as non-clustered. This probably isn't what you want to do, but I mention it because it is possible. Cluster members can have "non-monitored private" interfaces which are unique to the member.
You can also make a clustered interface as long as both firewalls see it as a bond. Bonds can be composed of a single interface, and the member with only one interface in the bond can do "round robin" with a single bonded interface. That will cause it to do no special negotiation on the interface (specifically, no LACP), it will just send traffic out directly. I deploy some of my firewalls' interfaces as single-member bonds simply because it makes it so easy to rearrange the traffic on the physical interfaces.
If you are particularly unaverse to risk, you can also run a cluster interface with differently-named interfaces on the different members (bond0 on one member, eth7 on the other). This is allowed by the UI and should work, but I guarantee it hasn't been tested to the degree I would feel even remotely comfortable using in production.
