I change them both because the labor is about the same to change one as it is to change both. The parts are about $45.00 each. If I have to make a trip back because I changed the wrong rectifier, I end up doing it at my expense because I didn't get it fixed right the first time. I suppose if I were an onsite tech for a company and they were paying me to be there 8 hours a day anyway, I might change one at a time. Since I have to drive out and change the rectifiers, it does not make sense for me to change just one.
Also, the customer wants me to make one trip and get his truck fixed right the first time. If I don't, he looses faith in me to do my job.
I have not found a reliable test for components in and SCR panel with intermittent problems other than changing the suspected parts and running the truck. I have found components the checked bad with a Handyman or VOM and work just fine. I have found components that checked good be bad. I have sent SCR panels out to be fixed and had them come back with more problems than they had to start with.