I had heard, a number of years ago, that there is a name for an type of idiomatic expression in which two things are joined to refer to one thing.
An example of this would be “raining cats and dogs”. This does not mean that cats is a metaphor for one type of rain, and dogs another, but rather that the combined terms form a joint expression with a single referent.
Likewise, “bread and butter” could be used similarly, e.g., “they should waste their time trying to sell frozen steaks; jewelry is QVC’s bread and butter”.
Is there a name for the specific type of idiomatic expression?