Depends how (and if) its wired. No joke - I had one on mine, but it had been replaced by a diode bridge and had no wires were attached.
I would guess you can start and charge with both batteries or take all the power (and insert all the charge) from/to one or the other. Normally, one battery is reserved for just starting the engine, in case you run the other flat, or if both are loaded, both flat - and then not be able to start the engine. I think there is one snag with a switch, depending on how it is wired. At no time while the alternator is running may it see "no battery" - without the load, it blows its electronics.