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Beide seems really complicated

beide Partys = both parties
die beiden Partys = the two parties

It really is that simple.

beide means always both! Zum Beispiel: Zwei Menschen gehen spazieren.Beide sehen ein Flugzeug. Beide hat die Bedeutung von "gemeinsam" oder "zusammen" !

Well, "both" can also mean "one or the other". :-)

"I would go to both parties" also means (AFAIK) "given the opportunity, I'd go to either party". The difference is that it's hypotitical, same as "Ich würde auf beide Partys gehen".

In the indicative form "Ich bin auf beide Partys gegangen" there's no such choice, so it can only mean "I went to exactly two parties", same as "I went to both parties".

So, "beide" always means "both".