Those switches are very simple inside.....it maybe just some corrosion on the connection terminal inside. I actually had one switch that had a bad spring on one of the detent balls that holds the switch in position and it wouldn't let it make contact on one position.
If it is indeed a bad switch then take it apart...you have nothing to loose!
One other thing you can try...take the positive batt connection off the switch on the battery that you are having problems with...then switch the move to the BOTH position....and see if there's an output on that terminal with the Voltage meter....If there is then would bet you have some corrosion on the part connected tot he handle and "should" be fairly easy to clean off with some emery cloth or light sand paper. Worse case its a burned and pitted spot and cant be repaired.

Rotate the battery switch rapidly back and forth through all positions and see if that improves voltage readings. If so you have corrosion inside switch as mentioned.

I'm with Sydwayz. Had same issue last year. Battery charged up and held voltage, but wouldn't turn the motor over. Took it out and had it checked. Found it was internally dead, cranking amps only showed 100 +. New battery and problem solved.

I understand and have observed the same but if it shows 12.5volts at the battery and switch imput...shouldn't it show something on the output when switched on?

Padraig

If that is the case, then I think it has to be the battery switch. Get a Perko switch.
You could try swapping the input cables to verify

Yes.

Thanks to all, specially SB...he called it. Replaced the switch today and it fixed the problem. Can start engine on either battery now. Dam....73 is too old to be squished into that engine compartment doing wireing and switch removal. These old knees were not happy. LOL. Thank God it is a single engine boat. I couldn't find a Perko locally so went with another Blue Seas.

Padraig