I teach tailwheel flying in a 1946 Aeronca Champ. It doesn't have an electrical system, but we use a battery powered intercom. Sometimes the battery goes dead and I have worked out a system of hand touches and signals so that we can keep flying, so yes sometimes we fly in a tandem without an intercom. I will admit that I prefer to fly with one. In the Champ from the back seat I can only see the airspeed and the tach (behind a large student I can't see any instruments). That means the student is responsible for the fuel gauge, altimeter, compass, and all engine gauges. I end up asking about things a lot. I cannot see the runway on short final unless we are slipping, which we fortunately do often. I have less viz for traffic. I really like teaching in the Champ because I like the aircraft, but the only advantage (other than being cool) that tandem seating has for instructing is that the student cannot see how closely you are guarding the controls.