ICRRE8, unfortunately cannot get to my DL's to confirm this theory. But I have a MTH set of E6's where the lead A-unit has and active rear coupler and so dose the trailing A-unit have an active front (facing rear) coupler. In this case there is a coupler selection switch on the bottom of the lead A-unit that allows either the rear lead unit or rear facing trailing unit coupler to function. It is a small slide switch marked for "on" or "off." When this "coupler lock switch" is in the off position the master (lead) engine rear coupler will fire. If set to on position then the trailing A-unit rear facing front coupler will fire.
check for this side switch probably to the side of the DL109 rear truck.
Just a guess since my E6's work this way.
Ron
ADDENDUM: Just pulled up pictures of my DL109 with cab removed. I see two side switches in the area beside rear truck. I do NOT see any reference to these switches in the Operator's Manual.