Common sense is what tells you the earth is flat. And yet, for most cases it's a pretty good approximation.
What is the intention behind disallowing realm mergers and strategic secessions? To enforce things that in the real world would not need enforcement. Very few kings would willingly stop being king. You didn't get to that job if you didn't have an ego too large to fit through a regular door.
But, we can not simulate reality that well. What you lose in the game is a fancy title and a bit of feel-good. That doesn't compare to not, you know, being the freaking king of France anymore. People have killed and died for just having a shot at that job. King routinely had their countries destroyed before even considering steping down. The common king, if asked if he'd rather die himself or see 50,000 of his subjects slaughtered, wouldn't even understand why you ask such a stupid question.
Realm mergers and secessions historically were almost always accompanied by warfare. Either before or after.
But, we can not simulate what it means to lose or gain the throne in the game. So we have to remind you that these are BIG decisions by pointing out that this purely strategic thinking is something you can do from up high, but very few people do when they are involved.
Now apply that philosophy to the case at hand. Here is what I believe would happen most likely if this were the real world: The weaker realm would split up, forced as it was to and not having much of a choice. The two parts would have friendly relations, only being hindered to re-unite because the allies forbid it. Over time, though, both would come to think of themselves as the only real successor realm, looking forward to the day they can re-absorb the other part. Of course, since both of them think that, there's a bit of conflict there. Then one day, the political climate changes, the iron curtain comes down, the people want to be re-united with their relatives and tear down the Berlin Wall... err... wait... wrong world. You see what I'm getting at?
Yes, under these special circumstances, a peaceful reunification would make sense, and thus not violate the spirit of the rule, !@#$ the words. However, there is also potential for conflict, because one of the parts will have to give up its identity, and hopefully neither will find that very easy to do. All it takes is one generation of players who came in after the split and never experienced the joint realm, and they will identify with their part, not with the old whole.
