Both persons will be mirroring each other's acts, poses, silliness, or facial expressions as if twins. One of the two will be usually holding the camera with your arms stretched out to take the picture (part of arm stretched out can usually be seen in pictures taken). At rare occasions, twinsies may be taken using a tripod or through the help of a passer-by.
"Oh really? I didn't notice that. Most couples are doing Twinsies these days!"
"Ya, not just couples, I saw friends doing that too now."