The States Admitted Side by Side

North Dakota and South Dakota were admitted to the Union simultaneously on November 2, 1889, when President Grover Cleveland signed the proclamations for both states at the same time, shuffling the papers so no one would know which state technically entered the Union first. The alphabetical order gives North Dakota the number 39 and South Dakota the number 40, but the true order of admission has never been officially established. Both states have occasionally quarreled over the distinction, though with a combined population smaller than Phoenix, Arizona, the stakes are largely symbolic.

Source: Wikipedia