If one deck of 52 cards can have 52! possible arrangements, how many can you get when you have two decks? If one deck of 52 cards can have 52! possible arrangements, how many can you get when you have two decks ?

You have now 104 cards, but they are paired. Assuming the two decks are identical, you have $\frac {104!}{2^{52}}$ combinations. Indeed there are $104!$ possible shuffles if the two decks are different, but since they are identical, for each pair of identical cards, you can swap them and you have the same shuffle.