From what you say it sounds like the thermal cut out is faulty.
Now each oven on this has its own cut out.
Look near the fan motor on the back in the bottom oven section and you will see the thermal cut out.
The easy way to test this is with a multi meter. But if you don't have a meter then you can bypass it.
join the 2 wires together and insulate them and then try the oven. If the thermal cut is faulty this way it will get hot an prove the faulty part.
Then you need to replace that. Don't be tempted to run like this all the time as the safety side is remove. Its ok for a short time to see if it gets hot like this.
Let me know how it goes.