I would only replace the one that is bad. That is what I did. I believe that people think if one failed then the other is close. I do not believe this is true with electronics. The one I replaced was 20 years old. The other one was going on 31 when the motor blew.
that is not tha reason for replacing both.. in some cases, not all, tha bad box can actually "cloak" itself making it it act and test like its actually good and tha good box look bad.. it does this thru tha bias circuit that is jumper-ed between tha two boxes..