Mac OS X v10.4 and later has a feature which allows you to combine multiple audio devices to work as a single device, called an Aggregate Device. This feature allows multiple audio interfaces to be used simultaneously by applications which do not have explicit support for using multiple devices. You can create an Aggregate Device with the "Open Aggregate Device Editor" command in the Audio menu of Audio MIDI Setup (AMS).

However, as of Digital Performer 4.0, DP has supported multiple hardware devices regardless of what verison of Mac OS X you are running it under. You can continue to use DP's built-in support for multiple drivers under OS 10.4 as you have previously, i.e., without using aggregate devices. Also, when using DP's built-in multiple driver support, you can quickly make changes to your clocking & sample rate settings without the need to open AMS.

For more info on using Digital Performer's built-in support for multiple drivers, see the tech note, Multiple Audio Interfaces In DP4.