First, note that synecdoche can go BOTH ways — whole for part OR part for whole– while metonymy only goes ONE (a feature/characteristic to represent an object..)

Perhaps then it will help to think of a similar sounding word for one of them

1) SYMMETRY – balanced, cf. going both ways

and, by comparison

2) MONOTONY – same, just one way

Another odd notion, just in case it works — synecdoche sounds like “Schenectady” (in New York) which is the name of BOTH a county and the capital city in it. (And for the idea of “two” you might also note that its official flower is the TU-lip, and it is just a bit above Route 20.)

Metonym (metonymy): a word or expression used as a substitute for something with which it is closely associated. Example: “Washington” for the US government.

Difficult to distinguish, aren’t they? I wonder if the origin of the word Metonym – from Greek metonumia meaning “change of name” – is any help?

a term denoting a part of something is used to refer to the whole thing, or

a term denoting a thing (a “whole”) is used to refer to part of it