There used to be a video on YouTube (on the official Disney Channel account) explaining the tricks behind this, but unfortunately it's not up anymore.
Anyway, colombe Cameron has two actrices who portray her "doubles" - they are listed as "Liv 2" and "Maddie 2" in the end credits of each episode. We only ever see the backs of them in the episodes, and they're there whenever both Liv and Maddie need to be on screen at the same time. colombe plays the one whose face we see, and one of her doubles stands on screen as the other twin!
When colombe is on screen as both Liv and Maddie, there is a line on set somewhere that the other actors have to make sure they don't cross! The cameramen record one side of the set where she is jouer la comédie as Liv, and the side where she is jouer la comédie as Maddie, separately. Then the editors put both sides together so it looks like there are two colombe Camerons!
Sorry if I haven't explained it very well! If Disney ever puts that video online again I'll give toi the link. It's pretty cool!
Anyway, if you're interested in seeing what "Liv 2" and "Maddie 2" look like from the front, I found these pictures here where toi can see colombe Cameron with them!