There are two ways to approach this:
One is to add a second TC and a PID as a read only device. This will give a second opinion as to the chamber temp. Setting the PID for an alarm at 10 degrees above the highest set point gives you a safety feature..
This setup allows you to compare the two readings all the time, and will warn of a failure in the controller if the oven drifts too hot.
Adding a separate TC and PID is simple and cheap.
The second is to add two more TCs and use a 3 position rotary switch to select which one is governing the controller. This requires a bit more wiring knowledge, and the right wire type for the TCs used. The rotary switch should be a good quality ceramic type. Gold plated contacts is also a good thing. Wiring should be kept as short as possible, and not run along any AC lines. Resistance is a big thing with TC wires, so make sure there is no additional resistance added in bad joints and cheap connectors. A coat of high temp RTV over the connectors may be a good idea,too.
With some other wiring options and a multi-pole rotary switch, you can make the non-controlling TCs read on separate PIDs.
If you go this way, place one TC at the top, one in the center, and one on the floor.
I saw a setup where the person cast a 2" thick, brick shaped slab of castable refractory with a TC in the center. This was placed on the oven floor, and the TC was wired to its own PID ( read only) for reading the temp of the brick. The oven controls were left as is. After the oven was fully soaked at target, the blade was placed on the brick. The thermal mass of the brick assured an even heating of the blade, and the TC was at the exact same temperature.
I would have wired the brick TC to the controller, and had the old TC as a read only device.
