ECN Forum Forums General Topics of the Trade General Discussion Area 3/8 "romex" connector use

Is a 3/8 "romex" connector listed to be used with a combination of a 12-2 and a 14-2 NM running through it? I believe that it has to be two 12-2's or two 14-2's (no mixing of different sized cables allowed). Of course other single cables can also be inserted.