[magno]

Do you have some kind of monitors plugged in to outputs 1 and 2 of your breakout box already?
Do you want the option of listening to monitors and headphones at the same time?
I think you need to explain exactly what your setup is now so we can tell you what you may need.
The easy solution tho is to find an omni i/o breakout box.
M-audio doesn't make them anymore but if you can find a used one you'll be happy.