Duocon could still be used as a reference term, describing a toy that has a similar concept to the two 'official' Duocons. They never had any story developed behind the Duocon concept in the comics or JP cartoon (they didn't appear in the US cartoon), so wouldn't it be interesting if the concept was an early experimental phase for combiner technology, or something else that Overlord was involved in and perfected for himself and a couple other Decepticons?

(just had a look at the tfwiki duocon article, and looks like Dreamwave did have the Duocon concept used as an early experiment for the Shockwave's Triplechanger project)

I think there is a slight difference with Overlord though, because he transforms to a base as well as a jet and tank, so really, he is more a triplechanger than a duocon.