If you have both the 3DS and Wii U versions of Smash Bros., you can link them together for a bit of cross-functionality. Do this by selecting "Wii U" on the 3DS game's menu and "3DS" on the Wii U game's menu.
You can use one or more 3DSes as controllers, but this will only work in modes found in the Smash menu. This means no Classic mode or events are playable with the 3DS.
Perhaps the most useful feature, if you've played a lot of one version before buying the other, is the ability to transfer custom characters from one version to the other. They'll carry with them any equipment and custom moves they're using. Characters can only be edited in the game that created them, though. Once a 3DS character is transferred to Wii U, it's stuck with its equipment and moves. However, it can still be edited on the 3DS and sent again.
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