With only one sound device, a user has no ability to cue a new track. In this situation, all sound sources are mixed into a single stream which goes to the device. Continuous mixing can only be done using the automated beatmatching system, which requires knowledge about each song used. With two sound devices, there are different ways to mix. The first method is to mix the room's sound on one device, and use the other channel for previewing new tracks, setting up loops and sequences, etc.
Streams are output to only one device, but which device can be switched on the fly.
This allows the user to preview a song playing alongside the current mix, then mute it, switch devices, and fade it into the main mix. However, be aware that sound output by the two sound cards will most likely be slightly out of synch. Thus, one will likely use the automatic beatmatching after switching devices. An alternate method is use an external, physical mixer to crossfade between two sound cards. One stream is assigned to each device, and new tracks are cued by making one channel audible only on the mixers' headphone output.