The good news is that you don't need to involve either AA. When you dial the extension that rings the local OBi, the InboundCallRoute would bridge to the remote OBi (via OBiTALK or a SIP service). The remote OBi's InboundCallRoute for that service would bridge to the Line port; you would then hear dial tone from the remote PBX and could dial an extension or outside call from there.
The bad news is that if the PABX systems don't provide CPC (interrupting loop current to signal a disconnect) and also don't play a disconnect tone (periodic beep), then the OBi will have no way to know when a call ends, other than detecting a long silence. This would result in the link being out of service for about one minute after each use.
If you have a spare analog trunk (C.O. line) port on each PBX, you could connect them to the Phone ports of the respective OBi devices. With proper configuration of PABX and OBi, you would dial a trunk access code on one system to hear dial tone from the other. There would be no issue with disconnect, as the originating PABX would simply hang up the trunk when the call ended. This setup would support two independent concurrent calls, one from location 1 to 2, and one from 2 to 1.
Depending on the routing flexibility of the PABX systems, you may be able to set up a unified dial plan. For example, extensions at location 1 would be in the range of 200-299; location 2 would use 300-399. If you dialed 312 from an extension at location 1, it would automatically connect via the OBi to the proper remote extension.
