On Sunday, April 5, 2020 at 3:36:09 AM UTC-4, John Foster wrote:

I'm no electrician, but wouldn't it work just fine if you wired both avionics and engine battery in parallel, with a switch (on-off) to each battery.. That would mean two switches. Turn the engine battery switch on before turning the avionics battery switch off. Or am I missing something here?

That often works, but the engine battery will be fast-charging the panel battery while both are connected in parallel, so you don't want to leave the switch in that position for very long. If there's a 5 amp fuse between the engine battery and the panel, you don't want to blow it.

John DeRosa's excellent tutorial details several good options for the two-battery issue. Starts at slide #95:
http://derosaweb.net/aviation/presen..._Practices.pdf

Cheers,
...david