Does Pro Tools 9 Support Aggregate Audio Devices in Mac OS X?
Mac OS X device aggregation of audio I/O hardware is not officially supported in Pro Tools 9. While certain configurations may work, they were not tested and qualified by the Pro Tools development team. The only exception to this rule is the “Pro Tools Aggregate Device” that is created upon launching Pro Tools to provide users of built-in Mac hardware with simultaneous record and playback capabilities.