I usually make it a practice to hide the mixture controls from the student with a large piece of cardboard or a manila folder. In this way, he cannot determine which engine has failed by glancing at the mixture-control levers. This technique gives the student access to the throttles and propeller-pitch controls, and he is forced to go through the procedures just as if the engine had genuinely failed.
Quote is from Schiff link. Evidence from surviving pilot revealed the instructor used this technique in the Camden Duchess fatal accident a few years back. Except he couldn't get the dead engine back to life fast enough before clipping trees and stalling into the ground.