The hookup is the same. My guess would be that the "every rider needs their own" is a liability sort of advice. There is no reason you couldn't run two different pairs of gloves on the same controller hooked to a bike, riding two-up. In a get-off, things might get slightly more complicated because of the unintentional unplugging of things, but that's no different than what would happen in a single get-off. Cords would get stretched to their limit and then the connection would disconnect.
Yeah, so I'd envision a setup like this: gloves for each, with the Y-cable, along with a coax extension (I see they have a 2-footer which would be more than adequate), controller in passenger's pocket.
**It wouldn't surprise me if the "one per rider" notice has as much to do with morons on different bikes plugging into the same controller as anything else. Standard "someone is eventually going to come along and do the stupidest thing we can't even think of" sort of disclaimer.