In order to unlock this achievement, you have to pot a double shot. If you have ever played Billiards you know what a bank shot is. This game defines 'bank shot' as a 'double shot', so all you have to do is successfully bank one of your balls. This can be done in multiplayer, or the 8 or 9 ball career. I have included a video, which can be used for the
They're Seeing Double achievement, so you can see exactly how it’s done.
Video credit goes to Jonathan Ingravalle.
I hope this solution helped! Leave a comment letting me know if there is any room for improvement! Thanks.