One of my switches broke in the off position on my dual pfo ballast a while back. I took the end of the case off that has the switches. I believe it is just 4 screws... And possibly one more on the side, I can't remember for sure.
That will allow you to take a look at the switches. The wires into the switches were pretty short, so you might not have much to work with, but I was able to bypass the switch and just wire the power from the power cord straight into the ballasts so that one is always on.
I use timers anyways, so the switches never get used.
If you know how to look at a switch and the lines and determine which to connect together to bypass, then try that first. If you are unsure how to do that, then get some help so you don't get shocked or worse!
Btw, the stuck switch looked fine on the outside, so don't expect to be able to look at it and tell if it is bad.