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I 've got the feeling they're both the same species.
I am not that familiar with the Nearctic fauna, but this looks like Terellia, in order to be sure I'll need a picture from above, for the wingvenation is needed for separation of the genera. There are two indigenous species: T.occidentalis and T. palposa. It is not one of the introduced Palearctic species: T. fuscicornis, T. ruficauda & T. virens.
I don't know what the difference between the two species is. But I can look into it.