To unlock the Twenty-Six achievement in Little Nightmares 2 you'll need to call out to your partner 26 times, you can do this at any point that your partner is with you. Here is my own video guide that shows how to get the Twenty-Six achievement along with all of the other collectible/achievement guides in Little Nightmares 2. The Twenty-six guide can be found at around the 1:50 mark during the video.