I get what you're saying - duplicity, back-up and all that, but running a hotspot on phone-A in an attempt to save battery while using phone-B doesn't fit my logic. Even with ~20% battery I can map/track for my typical rides with juice to spare (and that's not even managing cellular drain, just letting it run). My old phone netted me $300 off the new phone, which was a much better (everyday life technology) deal than spending it on "old" GPS tech. It gives me crazy battery life and for the next two years, as long as I bring a piece of it in, I get a free replacement. Win-win, cowboy logic, dunno?
Actually, it's more of save the battery of Phone-A (Primary phone I care about). That phone can sit in my backpack with hotspot on, GPS not running, screen off. It's the phone that I want to work when I'm done with the ride, use GPS to get back to my hotel or campsite and possibly not be able to charge it.
Phone-B (old crappy phone) is the one that sits on the handlebar running Trailforks/MTB Project getting it's wifi from the phone in my pack. Screen on full brightness and kept on so I can quickly look at it when getting to an intersection.
I use a Samsung S3 because it's worthless, it works for this purpose and I already have a handlebar mount.
The big thing with this too is if that Phone-B gets smashed or lost, I really don't care. I do if my primary phone does.