If they BOTH have a beef with Ronda, then they are both still involved in the same storyline. That's the point. It's not 2 seperate things... it's all still one angle.
And wrestling each other is not all they did for 4 months. Charlotte wrestled Ronda, and they both wrestled Asuka. Remember that Becky flat out COULDN'T wrestle for the past couple months or so, which required Charlotte to fill that void.
That aside, Beckys overness created the effect of fans booing the face she's working with. Inserting Charlotte protects the other face a little. Fans won't necessarily turn on Asuka or Ronda to the same degree if Charlotte is right there, since fans for whatever random reason seem to be doing that.