Diamond DA-42 Flight Instructor reported that; while transferring fuel; one engine nearly quit. After the fuel system was reconfigured the engine recovered; but a dual ECU failure remained; necessitating a precautionary landing.

Narrative

On routine intentional engine shutdown in flight of left hand engine; SP (Student Pilot) and IP (Flight Instructor) ran engine failure; engine troubleshooting; and engine securing checklist (simulating most of these steps). SP and IP noticed 3 gallon fuel imbalance (22 gallons in left and 19 gallons in right). To ensure that we did not go out of balance; IP instructed SP to pull RH fuel selector to crossfeed. This should pull fuel from left tank through fuel selector on right side and into the right engine. A few seconds after (enough time for IP to ask SP to pull out QRC to review lesson elements); RH engine started sputtering and began shutting down with a RH ECU (Engine Control Unit) A FAIL and RH ECU B FAIL. After that; IP took controls and fuel selector was selected back to 'on' position. RH engine came back to life but dual ECU failure remained. IP restarted LH engine appropriately in time. When opening up QRC; QRH; and AFM; we found no checklist for dual ECU failure. As a result; we contacted ZZZ approach and informed them that we were not an emergency but that we had a dual ECU failure and we returned to land. Rest of flight uneventful. QRC for the DA42 specifies that 'The remaining fuel in the tank of the secured engine can be used for the remaining engine to extend and maintain lateral balance by setting the FUEL SELECTOR of the remaining engine to the CROSSFEED position.'