Co-owners can be on the EC if a) the owner mention first on the roll nominates him or herself and b) if the other co-owner is nominated by a third party. Co-owners can’t nominate each other if they are both standing for election to the EC.
If they are legitimately elected, they have separate votes and they can both be office-bearers.
I think owners can nominate as many people as they want (with the above proviso) although I’ve never seen it done so I may be wrong.