Flight Instructor reported left engine loss of power with the right engine already shut down for training. The Flight Crew immediately restarted the right engine and performed an air turn back and precautionary landing at departure airport.

Narrative

I was conducting a training flight for a client working on a commercial multi engine add-on. When the event occurred; we were practicing engine-out procedures. The right engine had been intentionally rendered inoperative by turning off the right fuel selector; and as part of the engine-out procedure; the mixture; propeller; and throttle levers were full forward. The right engine was windmilling. While 'troubleshooting' the right engine; we felt a thump and experienced a sudden loss of thrust and drop in RPM from the left engine. I immediately took the controls and moved the right engine's fuel selector to the ON position. The right engine began running; and I began the engine out procedure.While troubleshooting the left engine; I determined that the left engine was still producing some power; though the manifold pressure could not be increased above about 11 inches; even with full throttle. The left carburetor heat lever was also stuck in the ON position; despite not having been on before the event. With the left engine still producing some power; I decided against feathering it. I [requested priority handling] with ZZZ Tower and returned for a normal landing. The landing; taxi; and shutdown were without incident. In the time following the initial loss of power; the student assisted me by reading checklists and double checking configuration settings.