If you ever add a 2ND elements, you need to add a 2ND of it too with the orange cord (fuse) and it means to drill a 2ND holes next to the first if or when you have a pid.
Are you talking about having to add a second high temp sensor when running two elements? That is not necessary. I'm running two elements on my OBS and have just the one original high temp sensor wired in series with both elements. Absolutely no need for two sensors.
I even removed the inline fuse and replaced it with one on the SG box where Bradley should have put it in the first place instead of having to remove 50 screws to check for a blown fuse.
The sensor in the back of the OBS (or the new one) that Toker is referring to is actually a thermal cut off. Basically if it reaches a certain temp if cuts power to the circuit until it cools down than it resets itself. The fuse that is mounted on the backside of the thermal cut off which you are referring to is not an electrical fuse, it's a thermal fuse. It acts as a secondary safety. If it reaches a certain temp it blows and cuts power to the circuit.
The main fuse in the smoke generator is the electrical fuse in case of a short in one of the circuits.
The reason Toker sets his up the way he does is because he wants the most safety devices he can have as he uses his indoors due to being physically unable to get outside to use it.