I’ve never heard of such fins. I guess I’m Bi-Curious.
I saw what you did there.
A Mono-Fin is a single fin where your feet are in that single fin in adjacent pockets. Bi-Fins are just a fin to each foot — the type you see in Sea Hunt reruns.
For free diving they use a very long rigid fin, perhaps stiffened with fiberglass structure.
Bi-fins means a separate fin on each foot, as opposed to a monofin, with both feet in it.
When it comes to freediving competition though, Bi-fin competition also means rules about not changing buoyancy (dropping weights or inflating a float) or pulling on the rope for assistance.