For ClusterXL, the IP that is always used to push policy to cluster members is the main IP of each cluster member.
This is listed in the General tab of the network object for each individual cluster member.
And yes, the policy is pushed to both members.
What interface that traffic will be sent on depends on the routing configuration, both on the gateways and the management.
Note that in R82 with ElasticXL, the policy will be pushed similar to how it operates on Maestro today (policy sent to the SMO IP, which replicates to other members).