The way I'm set up, one controller probe is in a large jar of water, the controller is set to 11°C and it runs the fridge. The second controller probe is taped to the carboy, the controller is set to whatever ferment temp I want and it runs the heater wrap. If things get too hot, the heater wrap shuts off and the fridge brings the temp down. If the fermenter gets too cold, the heater wrap switches on and warms the fermenter up. I set it up this way so that I could have different fermenters in the same fridge at different temperatures and still use the fridge to store finished beer in bottles. So far I've been able to bring the fermentation temperature up as high as 21°C and hold it there without any problem while maintaining temperature on the fridge. There are probably cheaper ways to do it but this works for me.
Hope this helps.