Have seen one used at a fairgrounds rv parking area. Power outlets were in pairs, with the idea of close parking for events shuch as rallies, dog shows, and othr events that attract lots of folks in rvs. 50A rig would pull in with a smart-ass driving, then take two 30A plugs using one of these adapters to use both plugs on one post. Ment that someone else didn't get a power hookup because these guys with their bigger rigs thought they deserved it. In that case, what they didn't know is that both the outlets were sharing the same 30A wiring.
I do have one, used with permission is a case there I knew that both 30A plugs were on different sets of breakers. Used to power both ACs in a very warm weather situation. Dealer offered to let me borrow it, then I bought it thinking that I might sometime need it again. Want one at my cost???