Roles (like calling "I got it!" in volleyball)

I'll start with the assumption that one guitar is carrying the bulk of the song and the other is adding color, texture, counterpoint, etc. This is a necessary starting point. It defines the roles for the song.

Voicing ("hummmm...me, me, me, me, me...." Not that. That's called vocalizing.)

Play different voicings of the chords. This is by far the most common thing to do. The easiest way to do that is by playing with a capo. So, for example, when one guitar is playing in the typical G formation, the other one can capo on the 5th fret and play in a D formation. Or get crazy and play with a capo on the 3rd fret and a cut capo on the 5th fret and play in D formation. But you can also play different voicings without a capo. For example, if you slide the classic C chord up two frets, it is now a D chord. Or if you slide a G chord up to the 8th fret (rather than the 3rd fret), it's a C chord. There are all sorts of possibilities if you'll just learn some basic ideas.

Playing it by the Numbers

Vary the number of notes played - one guitar picks, one strums. Or one strums on the downbeat or the root rhythm pattern while the other one strums more fully.

Take Five. (I don't mean the candy bar...though, they are delicious...milk chocolate, peanuts, caramel, peanut butter, and pretzels. Now this title is longer than the actual section. I digress.)

Get this. Ready? One guitar might only play on part of the song. Or part of the set. I know. That's just crazy talk.

"'cause I had to match the Rhythm of his belly with my head" (DC Talk, Jesus Freak)

The second guitar can play some complementary rhythm pattern, something offbeat, or syncopated that locks in with the main rhythm.

Riff (or for you highbrowers "Ostinato")

Think in terms of musical riffs - a pattern of notes played over and over with different chords underneath. It creates a counterpoint and the feel of pedal tones. This is different from continual "noodling" either on acoustic or electric guitar, which is never musical. I'm pretty sure that's not overstated. In fact, it may be that the defining line between amateur and advanced musicianship is the ability to use subtlety.

Happy playing!