No reason to have multiple armies
It seems to me after playing quite a few maps that levelling up mutlitple heroes/champions is counter-productive and having just 1 uber army that farms all the monsters is a much stronger strategy. Not only is splitting the champions difficult due to needing 2 strong armies (more upkeep costs and time costs for producing units), but it means you'll be much weaker in the long run due to being at a much lower level.
I think you should be able to let your sovereign have the option of training other champions when in a city to give some xp per turn (I know there is a building that does similar but it's very inefficient and luck based). Or perhaps even have any champion/sovereign get this ability under the Commander class (make it a bit stronger than it is now).
Alternatively allow champions to gain xp for managing cities, they could give certain boosts depending on the class. It gets to a point where you have 4/5 champions sitting around in cities doing absolutely nothing (they could explore, but you might as well send a scout to do that).