I would think there would be a range or mix of possibilities. I wouldn't consider most spouses to be fellow champion units, but I could see this happening occasionally. Seems like having a spouse unit as a champion all the time would be boring. I would like to see these possibilities for marriages/spouses-
Is fellow Soverign of neighboring Kingdom/marriage becomes alliance with some overlapping consolidation of Kingdoms>Kingdom=New name, bonuses, etc.
Spouse is champion or hero type unit; special unit with trade powers/powerful equipment
Spouse is in background; leaves all decision making to Soverign.
Spouse is behind the scenes and makes many important decisions or gives good advise. Bonuses.
Spouse is a liability and needs constant attention/more taxes/causes opposing faction(s) to slide towards being enemy combatants. Thinking of the two opposing families in Romeo and Juliet; also War of the Roses in Kingmaker, where Yorks vs. Lancasters.
I also see ways of ending marriage(s).
Beheadings. For the truly bad ending.
Divorce. Loss of some revenues.
Warfare. Yea, its suxxors.
Disease. Plague! Afflicts your Kingdom and is unkind.
Missing or lost due to quest. Could be kidnapping or spouse could be investigating a mystery.