Civ 4 had a mode something like this, but you still controlled your own cities (you were a team for things like diplomacy and victory).
Elemental would have to go a similar way, it'd be pretty confusing for two players to share a single sovereign. A "team" where you start off in permanent alliance makes more sense with the game mechanics.
But, I dont see why 2 Sovereigns couldn't govern one faction?
EDIT: My point is to share a faction, not a sovereign. 2 (or more) players would share resources, cities, borders, line of sight, units and design blueprints, well everything. The only thing that would be controlable by only one player would be sovereigns themselves, and perhaps their dynasties, rest is shared.