It COULD be the thermostat, but I think that is somewhat unlikely given the way this range operates. It could also be a half dozen other things. Like Phil said, the oven controls and their components for the right-hand oven on this range are VERY complicated. There is not only the selector switch involved, there is also an 11-volt oven control circuit that involves a transformer, 2 thermal relays and a thermal sensor. The thermostat for the right-hand oven is not your standard line voltage, capillary tube thermostat. Instead it is part of the low-voltage control circuitry.
The first thing I would check is make sure that the clock is set for "manual" oven operation. Beyond that, there is not much else that can be done other than by going "in" with diagnostic equipment and searching out the problem.
If there really is a problem in that control circuitry, it's going to take a good service guy to tease it out and to try and find parts to repair it. Another option would be to try and find a "regular" thermostat like the small oven uses and retrofit it into the large oven. This would make the oven operate but you would lose all of the "high end" features (tendermatic, self-cleaning, meal-minder, and the automatic broil control).
As an aside, I will mention that my grandmother had the Custom Deluxe 2-oven version of this stove when I was a child. She did 98% of her baking in the small oven. The only time I remember the large oven being used was on holidays when she was cooking for 20+.
I've attached the electrical schematic for the large oven here so that other members can examine it. Maybe someone else will have a suggestion for you.
