I have the TCW IBBS 6ah found here. There is a backup power master switch. Do you really need a separate switch here? I know you can't tie it to ground permanently as then your equipment will always be powered, but could I just connect this to the same switch as my main master? Note, I'm not wiring the pass-thru mode here in which you can skip this pin all together. Page 17 shows basically what I have setup.

I don't see a reason I can't wire them together. They are both just switches that connect to AF ground. You'd always want both on or both off. I can't see a reason you'd ever want one on and not the other.