King of Country A marries Queen of Country B. They have 2 sons. Does the oldest inherit both kingdoms?

This was a reason for so many dynastic marriages in the Middle Ages. The kids were viewed as chess-pieces to conquer, or at least get a foothold in, other countries. The Daughter of the Kingdom of Norway married the King of Denmark. Who got Norway as a wedding present.

It could go horribly wrong. The King of Spain, Phillip II, organised a marriage with Mary, Queen of England. He thought he'd got England in the bag as part of the Spanish Empire. Not so: the next Queen along, Elizabeth I, denied it applied to her, gave him the finger, and he sent the Armada, an invasion fleet, which was duly trashed by the English, who had no intention if submitting to Spanish rule...

In Europe, in the last 600 years or so, the eldest would inherit both kingdoms.

But: in the early-to-middle Middle Ages, when the rules of royal inheritance were still not fixed and every sovereign more or less decided for himself who would be his heir,it was quite usual for a king or emperor with more than one realm to parcel out his domains among his sons - e.g. William the Conqueror left Normandy to his eldest son and England to his second. In an age of personal rule and before modern transport, there was a purely practical aspect to this; if Vulcan and Ork are so physically separate that it takes a month's travel on horseback or a sea crossing that can't be made safely in winter to go from one to the other, then one monarch can't really control both, and everyone would be better off if they each have their own ruler.

In general the eldest would inherit both kingdoms.

It's not an identical analogy but King James VI of Scotland inherited the throne of England, even though he was a cousin, and became king of both which he then styled as the single Kingdom of Great Britain, ultimately leading to the UK as we know it today.

The oldest inherits everything. The two kingdoms would unite.

Yes indeed, that would be the case, old chap. You know, it would appear that both of our demises have been very greatly exaggerated, what what?