Reverse Invoke and outbound IP addresses

We are tentatively going to be using RI invoke in the following scenario:
DMZ Load Balancer balancing → 2 IS Boxes set up as RI servers in the DMZ → firewall protecting internal network → 2 IS boxes running Trading Networks in the internal network.

Is there some network configuration available which would allow outbound AS2 connections from the 2 internal IS boxes running Trading Networks to come from the same IP address? Some of our trading partners filter on IP address and we want to limit the IPs we use.

Thank you