The joint tenancy refers to those who are the tenants (i.e. a couple or other people co-occupying a specific property and sharing responsibility for the tenancy, and sharing rights from it) it is not a separate form of tenancy. Whether it is an AST or not should be detailed in the tenancy itself.
I'm more used to such arrangements being met with an individual AST for the room, with shared access to amenity detailed. I've not personally come across doing it this way - an immediate negative would be when the other person want to move on the remaining tenant would need a new tenancy, which seems the wrong way to do this.