Each sound will respond to a different note on the same midi channel so layering on a single track would be limited to 4 notes. Not so much of a problem.
Biggest thing you will miss is playing each ND channel chromatically, although you could just sample anything so.... another angle.
The ND2 is perfect with/for the OT.
Put it this way, no matter what you have, or how you use it, if you get the chance to buy an ND2 then you should!