Repeater stations not repeating the same LAN, but still need to talk to each other? Weird. To talk to each other through the WAN they don't need any special settings, but obviously they need routable IPs and no firewalls obstructing their way. The same default gateway that gets them out to the internet will get them to each other, though I guess it's possible that they could get different routes sometimes due to failures, delays, and traffic shaping.
Just to confirm: These are some sort of wireless (not necessarily wifi; I assume police stuff?) repeaters, and they are in range of each other on that wireless connection?
Can they be set with secondary IPs sharing a subnet on the wireless interface? For that matter, does the wireless interface already have an IP?