According to Nintendo’s Japanese Online Shop, Wii U GamePad’s are now available as a standalone purchase. To date, the only way to get a GamePad is with the original purchase of a Wii U or a repair order through Nintendo themselves.
The standalone GamePad is currently being listed for ¥13,824 which (as of today) equates to $112.90. This is $62.90 more than the Steam Controller and $37.10 cheaper than the new Xbox Elite Wireless Controller.
There is no indication (and no realistic chance) that dual-GamePad support will be coming to Wii U. The better guess is that this is available as a replacement option for those who break or lose their original GamePads.