King Valdemar 'The Wise' of Iceland needs your urgent help (Gavelkind woes)

UPDATE

Wow, thank you all so much for the many helpful replies! CK2 is such an enjoyable and detailed game, but it can be a little overwhelming at first, so great to see a community like this offering such wisdom and advice.

As you can see by the age of 60 King Valdemar's mind was made up. Whilst his firstborn Vestar was a brilliant strategist and source of aid in the many Viking raids and conquests that had allowed the Petty Kingdom of Iceland to remain independent of conquest and hopefully wealthy enough to remain so for the foreseeable future - his lesser son Vjorn had spent most of his life leading the distant armies of his Swedish wife and contributing nothing to the family.

And so it was with a decisive hand and a clear conscience that in the year 902 King Valdemar ordered the assassination of his son Vjorn (and obviously his one year old infant grandson) to secure the strongest inheritance and future of his realm.

First post on this glorious subreddit so greetings and thanks in advance if anyone can help me.

The Problem This is my first time playing under Gavelkind succession laws. I think I understand the basic gist of passing on the primary title to the primary heir and then dividing the subsequent titles and lands amongst the rest, but I'm struggling to comprehend how it directly relates in game and ultimately what I can do to control it or lessen the decay of my conquests (without resorting to gamey cheating methods if possible!)

The Scenario You can see King Ivar's problem here - upon death his lands will literally divide in two with Son A getting the Kingdom of Iceland (+ the 2 counties within) and Son B getting the Jarldom of Nordreyjar (+ 2 counties within)

I would be so grateful if someone could elaborate the reasoning behind the split in this way - it seems a little harsh on Son A who gets the same lands as his lesser brother - is he not supposed to a slight majority share as primary heir?

And finally is there any way to change or avoid this by granting land or revoking titles or do I just have to accept the fact that I am going to have two independent brothers with two counties each who will have to conquer one another through civil war and unite or risk being swallowed up by greedy neighbours?

Thanks!

P.s. Both this game and subreddit are all kinds of awesome - who knew murdering your children and looting Ireland to rubble could be so much fun