The saving doesn't apply if you buy both packs separately, unfortunately. On the flip side, anyone who buys both packs unlocks eight new colors of Yoshi and Shy Guy "immediately," in case you're in immediate need of some technicolor racing.
To recap, the two packs each contain three new characters, four karts and eight tracks comprised by two cups. The Zelda x Mario Kart 8 DLC (Joystiq alternate name: The Spirit Tracks) adds Link, Cat Peach and Tanooki Mario to the roster, with the neglected F-Zero franchise getting some love in the form of a Blue Falcon kart and at least one F-Zero track.
As for the Animal Crossing x Mario Kart 8 DLC (Joystiq alternate name: Animal Criss-Crossing), the extra characters are Isabelle, the Villager and Dry Bowser, while images show at least one Animal Crossing themed track.