I'm curious as to how twins were handled in the various succession customs around Europe in the middle ages, especially concerning primogeniture. Were surviving male twins a common enough occurrence that it was something that special rules were set aside for? Otherwise did the potential complications of twins make them an awkward development? Reading around on wikipedia its kind of difficult to find many examples of prominent twins but one case that seems like the kind of bad scenario I'm imagining is that of Ramon Berenguer II and his brother Berenguer Ramon II (creative names of course) who jointly became the counts of Barcelona but had a falling out that likely resulted in Berenguer Ramon murdering his brother. The fallout of that was civil war in Catalonia and the surviving brother ultimately resigning the post of count to his dead brother's son and living the rest of his life dogged by his reputation for fratricide.

Another pair that come up are Waleran and Robert de Beaumont, though their brotherly relationships seems an awful lot better than the Counts of Barcelona and they had a lot of influence in 12th century English and Norman politics.