You can use a single circuit, or two separate circuits.
A single circuit would require overcurrent protection of 80 amps. Wired with #3cu minimum if using NM cable.
If you run two circuits one would need 40 amps protection, the other 45.
14Kw is A LOT of electric heat!
I will STRONGLY suggest having this done by a professional!
Sometimes I feel like if I answer any more questions it is like someone trying to climb over a fence to jump off a bridge and me giving them a boost.
Answers based on the 2008 & 2011 NEC. If you're on the '14 already I feel sorry for you.