I have not seen the Charles combiner. I sell both Blue Seas and Victrom battery combiners. The Blue Seas is potted in epoxy and is virtually waterproof. I have never seen a failure for any reason. Blue Seas is very hard to beat.
My apologies, but I am just now giving this a second look. This MIGHT be a very good solution for this. It sure would decrease the amount of wires I need to build and won't increase the footprint of the battery switches I already have in place.Why not just use 2 A/B/Both switches.