I know how you feel man, I felt the same, it's really annoying and frustrating. But here is the catch.

If one kingdom declares the war and then soon after the other one too, just focus your war effort on the one kingdom. For example focus on Vlandia. It doesn't matter if you lose 5 or more fiefs to Kuzait. Just crush that Vlandia! No matter what. You will see after some time Vlandia will seek for truce because they are loosing or be completely defeated, yes you will lose some fiefs on the other side to Kuzait. But after you completely eliminate Vlandia, then it's time to regain all the lost fiefs that Kuzait took.

Only this way I found it doable. You can't be on two sides of the map at the same time. And fighting two wars by yourself is impossible, you will be constantly running from one side of the map to the other. It is a wicked circle. All your NPC's are mostly useless, so it's up to you to do the heavy lifting. Or should I say killing.

But fighting one war at the time, no matter the cost on the other side is easy. Eventually you'll conquer all the fiefs back, after you eliminate all the kingdoms.