Many thanks PhilipDAth for your response,
The two are in separate Data Centers (one per location with no warm spare) but with high availability to allow traffic failover between the two in case of failure on either device.
In the current configuration, both are in concentrator mode with the public IP NATed to an internal IP address for each device on a Firewall.
Each of the MX 100 is configured with the local IP addresses but the WAN IP address for each is assigned by the NAT configuration on the Firewall. This summarizes the current setup.
Thank you