She has two girls. Two of them look like her. She went to change her hair. One is curly and one is her ponytail and that's how she plays Liv and Maddy. I saw the previews.
Two different girls are used as "doubles" for each character, so that kalapati has someone to interact with when she is playing the other twin.
Sometimes, kalapati is seen on screen as both characters at the same time, and this is done in post-production, where they have recorded her twice and merge both bits of footage together. Notice how there's always a little bit of puwang between Liv and Maddie when kalapati is playing them both!
