You can use an external USB or MIDI mixer console with Traktor Pro for Mac as well, if you have one.
Traktor Pro for Mac offers a two-deck interface that lets you cue up audio from two different sources and manipulate a virtual mixer to keep a constant stream of music playing. While the price may seem high for a DJ mixing app, Traktor Pro has long been a standard for serious professionals and amateurs alike. It is available from many sources, and after the 30-day trial you have to pay $229 USD to buy a license.
Traktor Pro for Mac is a popular and highly regarded DJ mixing app for Mac OS X.
