You really can't combine them since you'll need to tie the tip/ring for each unit with the other. My money says that you'll end up causing both units to go and stay off-hook if you do this. Now since they are being fed from the same power supply, it might not be too dangerous to give it a quick try. I'm not a big fan of back-feeding a foreign voltage of any kind into a T/R output though.

You can certainly put two intercom buttons on the phones, one for each intercom and just tie all of the buzzer outputs together. The rotary phones will have to do their dialing on one button, or 'path' and the tone ones on the other. You just pick up on the lighted intercom path button to talk from either type of phone. You won't cause any harm by tying the buzzer outputs together since those are just derived by a matrix of dry relay contacts.

Ed Vaughn, MBSWWYPBX