Cappalli is correct. In a cluster the appliances will share the guest lic. You wont need any kind of HA license you just need to put the two CPPM in a cluster.
Couple notes:
1. You can have a VIP between the two appliance for captive portal failover.
2. For Guest registration to fail over between the two you MUST have a stand by publisher configured to take over incase of a fail of the publisher.
3. All guest accounts are created on the publisher so a publisher must be active to create new guests.