I want to use a Fernco coupling for 2 inch furnace exhaust is it okay? The venting is ABS and runs low across a doorway. I want to be able to bring stuff by and replace the pipe 2 times a year.
The short answer is that you'll have to consult your manufacturer's installation specifications.
Some furnace manufacturers specify only Schedule 40 PVC. However, some water heaters ship with couplings intended for use on the exhaust duct, so it's probably not accurate to say that flex couplings can never be used.
If you find that they're allowed, I'd probably use a shielded (metal-wrapped) coupling, especially since this is a horizontal application. You don't want the rubber carrying any load.