In my understanding, they both mean across, right? Are they interchangeable or used in specific situations?
There is a difference in meaning between these words. It would help if you could tell us the sentence that you are trying to use them in.
поперёк means "across" as opposed to "along", as in the expression "вдоль и поперёк", which literally means "along and across", and figuratively means "throughout" or "every which way".
Стать поперёк дороги - to stand across a road, like a car parked sideways blocking the road, or, figuratively, to stand in someone's way.
Через means "across" in the sense of "over".
Перейти через дорогу - to cross (walk across) the road.
Переплыть через реку - to swim across a river.
Через (preposition) has at least 6 senses in Толковый словарь РЯ (Russian language dictionary). It may be used to express the senses 'across, through, with, by, with the help (aid) of', in X minutes (when talking about time), so the idea of crossing is just one of the senses, but quite popular.
Поперёк can be both adverb and preposition. But it's basically just one narrower sense - across, more like spacial, geographical + metaphorical sense in set phrases and idioms.
Logically speaking, when you cross the 10 m width of a 10x500 m field you go поперёк. Whereas через could be any crossing - 10m across, 500m along, or one can cross it diagonally - all meaning going через поле.
So через and поперёк are interchangable only in certain cases, not always.