Polychords

Polychords are an example of Liberated Dissonance.

In music theory, a polychord consists of two or more chords, one on top of the other. The use of polychords may suggest bitonality or polytonality. Basically, Polychords are created when two guitarists play two different chords simultaneously, producing a new chord. In Jazz the upper structures are commonly played over a Dominant chord. Examples of a polychord:

Combining an Am triad (A-C-E) with a C triad (C-E-G) forms an Am7 chord (A-C-E-G). There are multiple such combinations that can be created.
Here's another: a C triad and an F triad forms an FMaj9 chord (F-A-C-E-G).
A Jazz example would be a G13(#11) Polychord: G7=[G,B,D,F] and A=[A,C#,E] G13(#11)=[G,B,D.F,A,C#,E]

Some compositional techniques are:

Tapping one triad with one hand, and another with the other.
Starting to sweep one arpeggio and then sliding up to a different (but complimentary) key for the second octave - just sweeping straight through. The sweep is quick enough that it's one fluid motion across both.

Slash (Hybrid) chords are very closely related to polychords. For example, G/A indicates a G major chord with an A note in the bass.

Polychords are a great cheat to easily play extended chords but they are a bad way of labeling chords. If you use them, always know what you are playing in relation to the root so you can understand the function of the chord.