"Both of them drink coffee."
Translation:Beide trinken Kaffee.
How about this, does this help? http://forum.wordreference.com/showthread.php?t=37546
Also, beide is a) a numeral, like in this case, b) an indefinite pronoun (when the numeral is used as subject). b) is often translated or at least interchangable with 'both of them', while a) is only 'both'.
Don't hesitate to ask, if you still feel insecure. I couldn't find any better rules myself.
I've found this rule online. Is it correct ?
If there is an article, it is with "n": die beiden Brücken.
if it is without article, it is without "n" but can have case endings: beide Brücken (nominative) der Standort beider Brücken (genitive) auf beiden Brücken (dative) über beide Brücken (akkusative)