Piggybacking Edirol

Yoy can have as many instances of Edirol running simultaneously as you want, only your computer sets the limit.
Are you thinking of doubling some melody lines or something?
English is not my native language, so I'm not familiar with the Piggyback expression.
If doubling is what you want, simply insert another Edirol, and copy all your miditracks, and let the copies play through this.
Or, if you want the doubling to be an octave higher, copy paste in the original miditrack, and transpose the copied events up.
Is this what you want to do?

Come to think of it, there's another way too, if I understood correctly.
Mix down the first group to audio, and play this back with the original midigroup.