i have 2 200w heaters in my 180 set at 80, but my Apex shuts them off at 78. i am glad that they are set up this way, because my Apex recently failed and the heaters were not shutting off. i lost a few things, but it could have been my whole tank. i had to send my 2 year old controller back in to get repaired.
the contacts in the heaters that turn them on get contaminated, then they arc and can get fused together causing constant contact. like stated above, its always best to use a second controller. the controller is far more cheaper than replacing the contents of your tank.