Warren in Lancaster, PA
It is not a bad idea to have a double iron rabbet plane. Peter Follansbee posted a picture of a double iron rabbet from maybe the 18th century. It had a loose double iron. I had seen a similar one earlier.
Frankly though, we rarely make a rabbet that shows enough that we care about surface quality. We do use rabbet planes a part of complex mouldings, but in that case we generally use near perfect timber and don't need the double iron.