For what its worth jigeye The method you described in Post #1 is an effective method of increasing either redundancy and/or duration. Yes its not perfect however it will work. I have setup a number of cross country airplanes over the years where the purpose in doing so was/is duration and not necessarily redundancy this especially so back before there were such things as battery/share devices and there were only Nicd's of relatively small capacity.
I have never used a batt/share device or felt the need for one. Now on the other end of the spectrum I have done two airplanes that are strictly single mission airplanes, and that is arrested landings. Those two (a Corsair and a Hellcat) spend their whole life doing nothing but continuous circuits with an occasional successful trap so I choose the most reliable engines ever a pair of OS 1.08's. These two are the only airplanes that I did feel the need for glow heaters because of the continuous go arounds so I used the same set up as your description except the second battery bypassed the Rx and only powered the heaters. I have also used the same set up with a separate battery for electric brakes.
So using that setup for redundancy or duration is effective and practical. Don,t be scared off by stories of doom. Total redundancy is of course an impossibility, using dual batteries/switchs without a batt/share while it will not provide total redundancy it will add one more step in redundancy over a simple single battery setup.
John