As astir says, , plus if the trainee is in the front you will prolly need to be quite finicky about your seating position. I have problems flying the supercub from the back with getting my feet nicely sorted between the brakes and rudder pedals. Side by side is easier for communication, and the student can see that you aren't riding the controls. Teaching turns is easier in a tandem, but for the rest I don't think it makes a lot of difference. If the solo seat is the back seat, a mirror to see the stude's face is handy. Not so many tandem light a/c around though, are there?