I'm with Steve on this one. we've had this discussion many times.
The Off-1-ALL-2 switches are not designed to go thru OFF from one position to the next. OFF is at the one end of the choices of positions. So you can't go "thru OFF to get to whatever".
And I agree with Steve that I have many hundreds of time switched from one bank to the other while the alt is charging and have never had a problem because of it.
Let me clarify my point: It is very likely that if you switch the system off while you're charging that you'll blow some diodes (and for the record, that doesn't mean that you'll "burn the altenator out"). But the switch does not go thru OFF to reach any other position. If you have a switch that can, change it ASAP.
Also loading up suddenly by connecting another flatter battery will not damage your alt. Think about your car. Switch on the lights, aircon, wipers and press the brake all at the same time. No problem. Even though this represents a significant spike in current. Alternators are not that sensitive. I can't remember how many times I have jump-started other cars from mine and then it's a case of connecting the cables to one battery then the other. The car that is running has a little surge as the alt catches up and then provides all the power you need.
It may be so that the switch may become "intermittent". This happens with everything from widgets to girlfriends. That is a extraordinary event. It will not do that 1 in every 10. It will do it once in a lifetime. Maybe. As Steve says and I agree, I have been switching these things from one bank to another for decades and guess what? I haven't had an intermittent switch yet (I have something else to look forward to). These switches are MADE to go from one bank to another. WHAT OTHER PURPOSE is there for them?
If you want to regulate your actions based on the extreme outside possibility that something could go wrong, you're likely to have an entertaining time sailing at sea.
Just my personal experience and opinion, as always