It depends i think, i have seen some pretty ordinary Anson and Deeley boxlocks in pairs or part of a pair. My own gun is no.2 of a pair, while it has nice engraving and wood, underneath its still a fairly standard A&D boxlock.

I also have a sidelock non-ejector thats one of a pair, you would imagine for guns meant for driven shooting, ejectors would be a must.