Well historical each Battle had several generals from different Nations .. You either have To allow much more leaders been active in battles or if the rule is really needed to use Johns Suggestion as many Axis leaders Huge forces of other Nations without having large own forces. ( rommel , kesselring , messe ).
i think in the Pacific there is also a us General that helped the Chinese .. You would Need a Special rule for him
I would not implement this rule .. I often find It enough unneeded complex that you can't move infantry of the other allied Nation in mech phase together with your tanks