The same answers apply here as in your first post but at least we have two images to look at now.
Here's a quick idea of 1 way to do it.
You need to place on image over the other (Layers) and then align the girls - Transform tool
Mask out the unwanted areas
Clone Stamp tool to replace areas the new girl didn't cover.
Adjust for lighting and color differences
I would like to see if you manage this, I'm fairly sure I couldn't do it. It's not the replacing the girl so much but more their poses holding the camera in opposite hands I think the background touch up would completely defeat me... Post a pick pf how you get on - Good Luck