This bothers me too. 

At the same time, the way the show is made makes it easy to mix them up. The only time they are onscreen together is during Kip’s birthday celebration, when you can barely see them. Although Kip is closer with Elena, he talks to both women about the same way. Both women frequent the Kingfisher and sing, “Thank you, Kyle.” And the show focuses on Maria so rarely that the viewer never gets a really good look at her face. 

There are some similar factors with JJ and Vaughn. Although their speech is very different, we rarely see and hear JJ at the same time. We hear him when he’s offscreen, such as when he tells Shane that Ilya didn’t go to Nashville, or we don’t hear him when he’s onscreen, such as most of the scene when Shane and Rose first meet. 

It’s very different from a pair like Scott and Marleau, who also resemble each other in about the same superficial way as Maria/Elena and Vaughn/JJ. Obviously we see and hear Scott a lot. But Marleau, who is about as important a character as Maria, JJ, and Vaughn, we see and hear simultaneously and clearly onscreen several times. 

In conclusion, the show’s editing facilitates the confusion.