Apple Logic Pro 8 - Manual (Page 216)

216 Chapter 10 Working With Instruments and Effects
3 Open the Mixer, and click the + button of the instrument channel that you have
inserted the multi output instrument in (Ultrabeat, for example).
Note: The + button only appears on multi output instrument channels.
An aux channel strip appears to the right of the instrument channel, already assigned
to the inserted multi output instrument.
4 Repeatedly click the + button to create more aux channels, for all stereo or mono
outputs available to the instrument plug-in.
You should only create as many aux channels as required for the number of outputs
used by the multi output instrument.
Following the creation of the first aux channel strip for your multi output instrument, a
– button appears beside the + button.
Click the – button to remove aux channels.