To unlock the X Best Friends achievement in Little Nightmares 2, you’ll need to locate the X-ray machine on the hospital level/chapter three and take an X-ray with Six.
The X-ray machine can be found shortly after interacting with the first TV in the hospital. Head right past the stairs and past an elevator into the X-ray room. You’ll see a handle, jump up and pull it and Six will run behind the screen. Quickly run around and join her before the X-ray is taken. I was holding hands with Six at the time, so I’m not sure if you have to hold hands with her to get the achievement to pop, but that’s what I did, and it worked for me.