What happens if a queen gives birth to a pair of Siamese twins?

Prince Charles would be crowned king, because the birth would almost certainly kill his mother.

However, your scenario is unlikely, on several grounds.

• The Queen is over 90 years old and therefore well past the menopause.
• There is no longer such a place as Siam. Prince Philip is Greek and any offspring are likely to be born in London. The twins would not therefore have Siamese nationality.
• Conjoined twins are vanishingly rare, and only half of those are male. Very few conjoined twins of either sex survive to adulthood.
• Having a pair of conjoined twins (two sets of two babies, making four in total) is implausible even in the same alternate universe where monkeys are current typing the complete works of Shakespeare.

I fan di Douglas Adams sapranno che ora dovremmo impegnare l'Infinite Improbability Drive.

EDIT: Ops, hai detto una regina, non La Regina. Colpa mia. Non altera molto le probabilità comunque.