Bitonality is the
technique of using two
keys (or 'tone centers') simultaneously in a
musical composition.
Stravinsky, Holst, and Villa-Lobos used this technique to a degree, along with lots of other 20th-century composers. However, Erik Satie's pupil Darius Milhaud and the American enfant terrible Charles Ives developed and utilized bitonality to an entirely different and larger degree. For an introduction to Bitonality - quite a mild effect by 20th-century standards - the piano works of Villa-Lobos serve rather charmingly, if less than profoundly at times. Polytonality, the logical outgrowth of bitonality, is the usage of three or more keys at the same time, God help us.
Bitonality's related to Bimodality, which is the usage of two or more modes based on the same tonic note simultaneously (most often the major/minor mode pair, otherwise known as Ionian/Aeolian). The absolute most famous example of this is the Major-Minor chords in Mahler's Sixth Symphony.
For an explanation of some usages of Bitonality in a Jazz context, see the URL http://www.tingjing.com/tg/articles/bitonal.html .