The original ones were made from a rack bar that happen to fit the non locking oval carabiners made at the time. Their use is,was frowned upon because that is the only thing used to rappel, no back up. They are for sale on eBay occasionally.
I don't think they generate enough friction for two people.
They should work the same for all devices and configurations, because it uses the tail of the rope, and doesn't interfere with what you're using.