You can use all instruments by clicking on the name of the first instrument and changing it.
But if you mean how to use more than 1 instruments with only 1 vestige, i didn't manage to do it, i think it's not possible.
Also remember that if you save a project with Edirol, Edirol don't remember the instrument you used in that proj, so you need to reset the instruments every time. Only for this thing is not so convenience, and if you don't like it, you can always try out DSK Plugins.
http://www.dskmusic.com/
Or searching for some orchestral samples or soundfont.