But the thing is... If they had been twins, then it wouldn't have mattered which one ascends to the throne, since they would have both been heirs of the dynasty.
And royal families from the time actively tried to have multiple boys, so that the succession would be secured even if some of them died(which children were likely to do before modern times)
So the French royal family would have never just dumped a spare heir into prison and obscurity out of some groundless fear that the younger twin migth try to ursurp his brother, especially not before his character was known. If he had proven to be problematic as he grew up, they would have just pressured him to take vows and become a Bishop (and maybe Pope)
It would be far more tempting for a twin to try something to depose his brother and take his place because of the resemblance. As for whom gets the throne, it definitely mattered a lot for the one who was going to be king. between the 12th century and the 16th (a troubled era), a great many pretendants to the throne were either poisoned, executed, or exiled for different reasons (mostly pretexts). Being the king of France was a highly disputed position and in a relatively small amount of time, many kings succeeded on that throne (Games Of Throne was right at least in that respect). Being a king was apparently a rewarding position but it was a dangerous one as well.