The Bobbsey Twins, of course! By Laura Lee Hope. Just beware, if you are buying the older books instead of re-issues, they had some questionable content. I recall one visit to the Southern states that had some terrible stereotypes.
I Love You the Purplest by Barbara Joosse is a beautifully illustrated picture book about a mom and her two sons and the lovely way she recognizes what is special about both of them. I don't think it says that they are twins, but they could be...
The Man in the Iron Mask by Alexandre Dumas
Are stories about look-alikes okay? The Prince and the Pauper by Charles Dickens
There's a book I read in the '70s called Me and the Terrible Two, about a girl who is dismayed when rambunctious twin boys move in next door (then ends up realizing they like her and are actually pretty fun). Otherwise, I can think of twins stuff, but it's mostly for adults or creepy!
There must be more twin books for kids -- it's such a fascinating idea ...
Two of the main characters in Spiderwick Chronicles are twins.
Also, there was a book I loved when I was a kid - it was from the 40's or 50's, I think, and was about a toddler boy whose mom didn't want to get his haircut, because he would lose his beautiful curls. The toddler's older siblings are twins. Sorry I can't remember the name!