davis is a PF and needs to stay as such
is our C and he's more than capable (if not always willing) to defend in the low post.
What we need from Davis is to bulk up and become a defensive and rebounding force, he can provide that from either position (PF and C). Bargnani
can focus on defending the big guys down low, which is his strength anyway, while davis can roam the paint challenging shots etc.
Offensively, andrea will probably evolve in a low-post threat over the next 2-3 years, if David can do the same, the combo will be lethal (it's already pretty powerful today).