You could connect 1 or 2 extensions from each Partner system to 1 or 2 CO ports on the opposite Partner, going both ways. Don't assign any lines to those extensions, and take the CO ports out of the Auto Line Select.
If you use more that one extension, put them in a Hunt group.
If I am at Business A, and want to transfer a call to Business B, I simply press Transfer and dial the extension number or hunt group number (or press an intercom auto dial button) and the call rings in on the CO buton(s) at Business B.
If I am at Business A and want to talk to a particular extension at Business B, I would press the CO button that gives me intercom dial tone from Business B and dial the Extension Number.
Curious: would a single Partner system have enough CO and Extension ports to simply have ONE control unit, and run a 25 or 50 pair cable between buildings?