At that age, I would suggest just one. Many times, a book is good for a shy child to use as an ice breaker to approach characters. If they get the hang of autographs, they might take turns holding the book. One is easier to manage at that age. However, since they are not that expensive, you can bring a 2nd book just in case you need it halfway through the trip.
With my 2 boys, ages 6 and 3 on our October trip, my DS3 was not at all into the autographs. My DS6 was and he managed the book for us. DS3 liked to look at the book at lunch breaks and back at the hotel, but otherwise could have cared less. He loved interacting with the characters, was not shy about approaching or playing with them. He wanted to play with the characters instead of getting them to sign some paper.