The two avatars can only know 8 main elements(including Energy). In total, there are 14 main elements. In order for this to be fair, each avatar has to know different elements than the other. This means if one avatar knows dark, the other can't. Avatars are also allowed to have one sub element from an element they don't know. Say an avatar doesn't know water, this rule allows the avatar to be able to use one if its subs(Ice/Blood). Both avatars are required to have "Energy Bending" once they reach the "Titan" rank.

Elements Known:

Auron:
Water
Dark
Air
Sand
Acid

Future Elements

Plant
Body
Energy

Elements Known:

Nazeria-
Fire
Earth