Bonjour Kay,

In this context (finding seats together), we don't use the word 'siège' but 'place' (it comes from 'place de cinéma' = 'cinema ticket'). However, you can still use 'siège' when referring to the seat you are sitting on in a cinema when it is uncomfortable for example:

mon siège est inconfortable = my seat is uncomfortable

I hope this is helpful.

Bonne journée !