I haven't seen anywhere in the guide or the forum anything that says 2 heroes can't marry. In my previous kingdom, I had about 4 couples get married. But in my current kingdom, the monarch (Angelina) and the Knight (Alred) hit it off almost on their own. They started 'Romantic' conversations even before I was out of the 'Friendly' mode!
Their relationship bar is maxed out, they are engaged and already have 2 kids, but every time I select the Have Marriage option (via the Monarch), I get a message that 'Someone has gotten cold feet', the action goes away and the knight leaves. This has happened twice, but it still hasn't stopped their almost automatic WooHooing, which definitely gets in the way of the Monarch's quests when she gets pregnant!
All the previous marriages were between a hero and a game character. So I didn't know if 2 heroes marrying was a problem or it's something else that I'm missing about the relationship!
Linda