How to incorporate 2 guitars into a song which uses only 1 guitar?

There are a couple of ways you can do this simply. You can have one guitar playing open chords while the other plays barre chords. You can have one leading and the other accenting. You can write a riff for the second guitar to play. You can have the second guitar pick the root notes. You have one guitarist playing acoustic and one play electric to add a different sound. Also if you cant find a way to use both guitars one of the things we've done before is had our rhythm guitarist play hand drums for that one song. You have al ot of options, you just have to figure out what will work best for the song your playing. I hope this helps, good luck.

if you're talking about a cover song then you can have both guitars playing the same thing (ex. avenged sevenfold has parts in some of their songs where one guitar is used but live they both play the same chords/riff like in the main riff of "critical acclaim")
live vid. of a7x "critical acclaim":
http://www.youtube.com/watch?v=wDtrYMCCM…

if it's a song you wrote then you could probably add another part to it.

or the third option is to take the creative road if your doing a cover song and add another part to make it your own (a good example of this is the rock cover version of "apologize" (covered by silverstein))
for this you could add a completely different part or add different chord voicings (where two chords composed of the same notes but in either a different position or place on the neck)
vid. for "apologize" cover:
http://www.youtube.com/watch?v=q8CxOA1D7…

ecample of a jazz chord voicing for Gmaj7:

type a type b
e|-------------|-------|
b|----3-------|---12-|
g|----4-------|---11-|
d|----4-------|---12-|
a|----x-------|---10-|
e|----3-------|--------|

and one guitar would play type a and the other would play type b.
you can also play power chords over major and minor chords.

another thing i can think of is if one guitar plays a power chord then you can add an octave chord to make the technical major or minor chord.

example:
PC OC
e|---------|---------|
b|---------|---------|
g|----5---|----9---|
d|----5---|----x---|
a|----3---|----7---|
e|---------|---------|

the PC (power chord) is a C5 and the octave is an E. together they complete the arpeggio for a Cmajor. When you do this it helps to stay in the key or the progression won't sound right.
also in "critical acclaim"

the final choice for lead parts is harmonizing (i'm not demonstrating that).
lesson on harmony:
http://www.ultimate-guitar.com/lessons/g…

well it depends on what sort of song it is and what your playing (electric acoustic etc) but one creative way to add a bit of flavour woulb maybe be too have on of you play the rythm and the other to play plucked notes and come up with a melody to go with the rythm part. (an Arpeggio)
good luck!

all good answers so far, , , i will add that no one else said
you can tune one a coulpla cents up or down, down is generally better, this will make a natural chorus, it is hard to do,lol,, but it is worth it when it is done right,
by cent i mean there are 100 points in-between each half step(one fret) try using one at around 97, as oppoused to 100 which is regular pitch (A 440)
there is always not playing the same time too,
and registers one play top and the other bottom \
really its endless of what you can do and my personal fav that is the easiest is to close your eyes and bleed and bounce off of each other; i do mean musically,lol,,
really it is endless if you think about it

You could harmonize if you know how to. But if not 1 guitar could pick the root notes of the chords your playing. For example:

If the chord being played by guitar 1 is
---77777777---
---77777777---
---55555555---

Other guitar can play

-------------------
------7--7-7
---55-55-5

Simple and it sounds really good.

If they have different enough tones, try having them play the same exact thing, It could add dimension...

One guitarist can play rhythum and the other lead....

have one guitar playing an octave higher, dont pick me for best answer that was sh*t