There's another technique (only suitable for those with light strings or heavy fingers!) called "unison bends". You play a note on the upper string and play a semitone below it on the string below, and then bend the lower string up until they're the same pitch.
So, for example, play A (5th fret, top E string) and G (8th fret, B string) at the same time, and bend that G up until it reaches A.
It's a popular rock guitar technique - Carlos Santana uses it to great effect in Samba Pa Ti. It's possible on acoustic guitar, but it doesn't have the same effect. It's mainly suited for those moments where you want to really emphasise a single long note - there's no way on earth you could play an entire song like that!