I'm also curious about other countries as well/good historical examples.
One important thing to note about the "feudal" system is how inconsistent it was, because, in fact, there was no "feudal" system. Different areas could have different laws, rights and obligations even within one country, let alone different countries. In addition, in the early Middle Ages laws were often uncodified and based on multiple traditions, especially in the British Isles. Who inherited titles in fringe cases was very much something that was often poorly codified, or could change over time depending on parliament, and the opinions of the power-players. A good example is how when Matilda, who had no brothers, inherited the Crown as her father Henry intended; England was immediately wracked by civil war as the nobles rose up to put a man on the throne (the question of whether a woman could inherit was not the only reason for this, but it was the justification).
So, in England, there was no particular law about what would happen if twins were born. Presumably, the first to come out would be King, but it never happened.
However, it did happen once in Scotland. Alexander, son of James I, was born first, becoming the crown prince while his twin James was born second and so was not the crown prince. However, Alexander died as a baby, so James succeeded him as crown prince and later became King James II of Scotland in 1437 (at age 6).
On the other hand, an example that wasn't so clear cut were the twin Counts of Barcelona, Ramon Berenguer and Berenguer Ramon. When their father died in 1075, they were supposed to rule jointly, according to their father's will. But they couldn't agree, and split their possessions up instead. In 1082, Ramon Berenguer was killed in a suspicious hunting accident, and Berenguer Ramon inherited his land. So you can see that medieval succession law was quite varied, so that in one place twins might rule jointly or divide the land, while in another the oldest twin was simply the heir. Of course, this is also connected to the time: in the early and middle medieval period, splitting land between your sons was reasonably common, while as the period went on giving all your land to your oldest son became more popular.
To add onto Manchurian's point, yes, English succession laws in general were much more about what claim you could make rather than the claims you were born with. A good example here is the complicated situation in 1034 when King Cnut, the Dane who held the English throne, died.
Cnut had the English crown as part of his North Sea Empire through his father, who conquered England and took the title from Aethelred the Ill-Advised. Aethelred had been married to a very well-connected noblewoman named Emma, and they had two sons, Edward and Alfred. When Cnut inherited the kingdom, he was already married to Aelfgifu of Northampton, a woman from an affluent Mercian family, and they had two sons, Swein and Harold Harefoot. Despite his own laws against polygamy, Cnut also married the widowed Emma. Together, Cnut and Emma had one son, little Harthacnut.
If you're as confused by all that as I was when I was first learning all this, you're probably in the same situation as most of the English people at the time. But in short, you have about five sons with varying claims on the throne. Edward and Alfred had a claim through their deposed father, and their mother was in favor. Swein and Harold were the firstborn sons of the current king. Harthacnut was youngest, but he was the 'most legitimate' in that he was the first son of the newest royal couple.
So when Cnut died, a propaganda war broke out. Emma and Aelfgifu pressed each of their sons' claims, trying to curry favor with their nobles. Aelfgifu doled out gifts liberally to the nobility, Emma claimed Aelfgifu's sons were bastards, and Harold Harefoot may have even forged a letter to lure Alfred and Edward into a trap that ended with Alfred getting killed. In the end, though Swein died of unknown causes, Aelfgifu's efforts secured the throne for Harold Harefoot, whose short reign was followed by Harthacnut, and only after him did Edward get the throne, going on to become Edward the Confessor.
In short, royal family ties were usually very complicated, and when a monarch died, a lot went up in the air. Instances like William the Conqueror, where one ruler with a clear series of sons is in power and the rest of the table is wiped clean, were the exception rather than the rule. Some of that has to do with the fact that power was much more decentralized in the early/high middle ages than it was in the 14th/15th centuries. As was the case with figures like Matilda, established succession or royal wishes commonly got overridden by the struggles within the nobility, and whoever had the better propaganda machine had the final say.
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