Just to add a bit of context here, the expected setup for an MX HA Pair is Master-Passive, effectively meaning the two physical appliances are sharing a single logical setup (with exceptions in the uplink IP address). As such, only one license is required for an HA pair. If you want to have the appliances in different networks, at that point you require separate licenses.
It's also worth re-iterating @rhbirkelunds point that we do not support Active-Active HA pairs. There may be times when both appliances show as Master, indicating that they are failing to exchange VRRP packets. In those situations, you are more likely to experience issues with Internet traffic as your downstream LAN may be flapping between two uplinks.
Please have a read of our documentation for some more context and let us know if there are any further questions 😀
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