It is possible for the Wii U to support two of its touchscreen controllers at once. But Kit Ellis, a spokesperson for Nintendo of America has said:
We do not have any such plans right now, but in the future we could explore how two controllers might be used in game play and what new experiences that might introduce. We will make such announcements (about selling controllers separately) later in each territory.
And that’s a bit of a bummer, although it’s not unexpected. There’s so much potential in how two controllers could be used, so at least they’re thinking about it.