This is one of those rules that I hate. Back when I was a cave diver (probably the most dangerous hobby one can have) there were basic rules. You always had three light sources, but only one set of tanks that could be isolated from each other. Two regulators, but that was to support your buddy in case of emergency. Everything else was singular including cave reel. My point, you made sure that your gear was always perfect, you were streamlined so as to not catch anything, and you were trained above and beyond what normal divers do.

Carrying backups is fine in some situations, but they do add up in weight and bulk which causes additional stress on the body and starts to kill your performance.

My EDC, Ritter mini, streamlight (CR123), bic lighter, 4 quarters, mini fisher space pen, and my phone. Photon on my key chain, but I don't always have them with me. Firesteel with tinder in my parka pocket, but only if I wear it.