You could create an IAX2 or SIP trunk between the PBXs. Then create an outbound route on Server 1 with the Pattern XX and on Server 2 another outbound route with the pattern 1[0-5]X.
Additionally, you can set both outbound routes as “Intra-Company.” This will makes sure that the CID sent over the IAX2 or SIP trunk you created through the outbound routes to each PBX is the Internal CID, not the External CID.
Another thing you need to make sure that you also have inbound routes to the extensions with the extension number. Since the call is coming from another PBX, the second PBX needs to know where to route this call.