Intracompany IAX : Ring Group Issue

Hi have two PBX’s setup with an IAX trunk between them.

Users can call from one side to another and that works fine.

I am having an issue with a ring group on PBX1. I want to include another ringgroup on PBX2.

So when the group is rang on PBX1 (640) it rings local agents and a ring group on PBX2 (600). However PBX1 detects the call as being answered and hands over to PBX2 and stops ringing any local agents.
The ring group on PBX2 does not have music on hold, it is set to Ring.

How can I solve this issue?