Its easy to do if you're on a course with two FOs. As one will always be playing captain
It seems logical that a candidate undergoing a command endorsement from the RH seat should make all the decisions including a decision to abort a take off. This leaves the occupant of the LH seat as nothing more than support pilot (PNF) and not the captain for the exercise? He should therefore not be taking over control from the command endorsement candidate to reject the take off following engine failure.
The fact that airline SOP generally requires the captain (LH seat) to conduct the rejected take off procedure has no relevance to training for a command endorsement from the RH seat. If the command endorsement is conducted from the right seat then all decisions relating to the flight including the decision and physical actions to reject a take off below V1 is the responsibility of the RH seat pilot.