I think the diode idea is a poor way to go unless there is full time
monitoring of both of both/all batteries. If one battery goes bad the
other will be discharged before you know you have a problem. This
solution offers no more redundancy than a single battery with twice the
capacity. With switched batteries the pilot has the option to minimize
current draw to conserve the second battery for essential equipment.
Short term power loss does not invalidate the flight log.

With a 302 the ideal solution is probably 2 switched main batteries and
a backup battery (factory option) for the 302 only.

I'm wired for 3 switched batteries but am only using 2.


Andy