Still, chains wear out and become loose and 'stretched' over time, and can and will flop around and sway a bit. More so as the chains get older and worn out.

On the bike in the post, there doesn't seem to be any automatic chain tensioner, so as those chains wear out and loosen up more, they can and likely will sway enough to bump into each other.

When you consider the already existing hazards that can cause a chain to break, it seems to me this just adds more potential failure points, plus a failure mode where the chains can interfere no less, which isn't even a failure mode on a single chain bike.

There's already plenty of other gearing systems for bikes that only require one chain. Sure, the whole derailleur system can be finicky and easily damaged, but there's also the 3 speed hubs, where there's basically a 3 speed gearbox built right into the hub, eliminating the externally exposed gears.

And with the thought of the 3 speed hub (they also make them with more gears than that even), it just seems this dual chain system is a solution to a problem that didn't exist.