At each PBX, set the context of the trunk between the two sites to “from-internal”. On the outbound routes used to route calls between the systems, set the Route Type to “Intra-Company.”
No. Routes do not usually have custom contexts. On either end of the trunk that connects the two systems together, on the trunk’s advance settings tab, set the context to “from-internal.” This assumes you are using chan_pjsip for your trunks. If you are using chan_sip, I don’t remember where you set that.
This is assuming you have set up a dedicated trunk between the two systems to handle calls between the two and have routes that allow dialing between the two systems.