Siamese twin

noun3

1. •
   A conjoined twin; one of two people physically joined together.
2. •
   One of two people who are extraordinarily close, or always found together.

   • Didn't you hear what he said about your Lesta and the Prince — that they were as thick as butter — regular Siamese twins and you couldn't separate 'em?
3. •
   Either of a pair of words that occur together as an idiomatic expression or collocation, as in "hammer and sickle", "short and sweet", or "spick and span".