There are several ways to do this. Easiest is a PRI link between the two if you have capacity to expand. Straight up programming, just create the routes and set up destination codes to use the PRI or auto-answer T1 E&M to route the calls to the Intertel. For inbound you'd need to set up target lines matching the extensions on your BCM and assign them to the phones.
If PRI/DTI isn't possible, you can use analog station ports off each sytem appearing as analog trunks on the other system. Put them in a line pool and when you want to dial the other system, just dial the pool code plus the station number. This would be a FXS to FXO configuration.
You could also use SIP but that may be a long shot a require SIP gateways.