Because the two copies must breath air, the two would instantly become physically different.
This is because, although their lungs may be identical to start with, the air they breath will contain different distributions of oxygen atoms and therefore their bodies would absorb different numbers of oxygen atoms the moment they breath.
Thus, they are identifiable as two separate individuals and would act accordingly.
EDIT See my comments below, in reply to your comments. I should have edited these comments into this answer.
EDIT As per you request, I have copied my comments into my answer.
How one answers this depends on whether one believes that reality is entirely material and determinate. If you hold such a view, then in the absence of any external forces all of the trajectories and the observable behaviour of the particles which make up the two copies would continue to evolve in exactly the same manner in accordance with the applicable deterministic laws. Therefore, any action taken by one of the copies would simultaneously be taken by the other. They would act in exact unison - saying “hello” simultaneously and continuing in unison.
On the other hand, if you do not believe that reality is material and deterministic, then there is room for variance in the absence of external forces. For example, free will, chance, etc..