Two different girls are used as "doubles" for each character, so that ঘুঘু has someone to interact with when she is playing the other twin.
Sometimes, ঘুঘু 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 মহাকাশ between Liv and Maddie when ঘুঘু is playing them both!
Here's the প্রিভিউ video:
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