I realize the question is a bit old, but I'll go ahead and answer it. :)
Yes, you can do this, and you use the upstart "instance" clause to do it.
Here is a simple example (using a PHP based job in this case).
description "Async insert workers"
author "Mike Grunder"
# This is the key bit
If you see in that script, you've got an
instance $N line. You specify the instance like so:
start the-job N=instance1
start the-job N=instance2
Obviously you could change
N to whatever you like.
Now I'm back on my quest to figure out how to start a number of them automatically! :)