All ClusterXL HA interfaces defined as "Cluster" where a Cluster/Virtual IP is being presented have essentially equal priority. The ClusterXL HA cluster member with the most working interfaces will "win" via CCP and go active. You can define an interface as Private and therefore non-monitored, but I don't think there is any way to present a Cluster/Virtual IP on an interface defined that way. You might be able to play some games with proxy ARP on the Private interface though.
Your request can be done with VRRP however using different priority deltas. For a non-critical interface define a low priority delta that upon failure will not degrade the effective priority of the Master below the base priority of the Backup. However if another interface now fails on the Master, that interface's priority delta will be enough to drop the effective priority of the Master below the base priority of the Backup, and a full failover will occur (assuming you have set up monitored circuits correctly).
Generally I try to avoid VRRP in favor of ClusterXL though, with VRRP it is way too easy to cause split-brains and routing back holes if everything is not set up 100% correctly.
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