Supplying two temp controllers with a single element thermocouple
It would work but affect the accuracy in two ways. 

Depending on how you hook it up, it would throw off the cold junction compensation. If the split happens outside of the controllers. A good controller will monitor the temperature of the TC terminals for the compensation. 

It would also affect the loading. Typically the controller is compensated for its internal load on the TC. If you put two controllers in parallel the load is effectively double. This will cause inaccuracy in the reading. 

Depending on your controller it may be possible to calibrate out both of those effects by swinging the TC through its range in an oil bath and then storing those points in the controller. Most likely the controller just accepts a specific type of TC and is precompensated for it.