It won't be until next year that the first Wii U titles supporting dual GamePads and subsequently Nintendo won't be selling GamePads separate of the consoles bundles until that time. Probably a wise move as we could imagine confused consumers getting an extra one just to realise there is zero reason to get one at launch.
No future games have been confirmed to support dual GamePads at this point, but hopefully we'll hear of some in the near future.