How will you celebrate their birthday and which year will they claim?
I have heard this kind of scenarios many times, it becomes a matter of choice for the parents when to celebrate the birthday of their twins. Children are not called twins only because they share the same birthdays, birthdays did not even matter at some point, Children are called twins because they shared the same space in their mother's womb. To celebrate their birthdays will become a decision but the best day to use is the day the first child came out. This case is also very similar to a case I have seen about children born on leap year dates like 29th of February and which date they will also have to choose to celebrate on in years that are not leap years.