A whole lot of information isn't readily available on this evolution, but the official site says:
What appeared to be a single blade becomes two when Honedge evolves into Doublade.
"The two swords converse telepathically and can carry out complicated attacks together."
Doublade will also be one of the few to learn the move "Sacred Sword" which is not affected by stat changes on the target and is a fighting type move.
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