Regulators come in two stages; you carry a backup second stage for someone else if their system fails - but you don’t carry two first stages (the part that screws onto the tank).
While it is used primarily in cave diving because it is more streamlined and less likely to collide with cave ceilings and features, I find it also quite comfortable for recreational dives with a pair of AL40s.
It’s also a fun conversation starter with other divers who are curious about my gear!
You get air tanks that have 2 mounts for first stage and the octo goes into the separate first stage, giving you two independent setups.
I've only seen crazy swiss people do that though.