double with manifold is one redundant system and it s perfectly fine, sidemount is another and it'sYou didn't say what your other reasons were for switching to sidemount, but the one you did mention is redundancy. I'd like to propose that manifolded (backmount) doubles give better redundancy than sidemount. With manifolded doubles, if one regulator fails, you can easily and safely access all of the gas from both tanks using the remaining (working) regulator, without resorting to inconveniences like feathering valves.
perfectly fine also. Not one is better than the other. The system should fit you need and the dive you would like to accomplish.
some of us are getting old and are going in a wheel chair. Sidemount is better for me. When i was young and beautiful i could do double not anymore. My back and knees are over.
Happy life to sidemount for that matter i put the tank in the water and i don't carry them on my back.
Manifold are nice like you described them, advantage to sidemount the valve are right there less chance of doing a mistake in case of emergency when you heart in pounding like crazy tunnel vision kick in. SM you still have two independent system that are available for you. It's a valid redundancy system.
Either in sidemount or double you will have plan your dive and prepare to have a reserve. If a major issue arise you will abort the dive anyway and make it home with the remaining gas.
I think these guys explain it better than me. Not related to a regulator malfunction but pretty close to the subject and the way i see it.
So the statement that double give better redundancy than sidemount is not accurate,
proper training will teach you either on double or sidemount to prepare for an emergency.
Be safe