I asked a similar question here:
The Cantonese title of the film Us has been translated as:
Wikipedia also mentions that:
So, it seems that the Cantonese title is quite different here.
Guangzhouhua-Putonghua only mentions Mandarin definitions.
While Words.HK only gives the following:
ji6 hau2 tung4 sing1
saying something together; literally, different mouths but same voice
I'm still not getting the pun, though.