The official movie website has updated with details on the upcoming 2-hour special. Translations follow:

In order to commemorate the premiere of "Pokémon the Movie XY", we will show "Extremespeed Genesect: The Awakening of Mewtwo" and world exclusive footage of Diancie together in a single broadcast!
They will be shown as a 2-hour special airing Thursday July 17th!!
It will be shown nationwide on the TV Tokyo Network, as well as on various other channels.


The host of this special will be Mayu Watanabe, who also serves as the narrator of "Pikachu, What Kind of Keys are These?", the movie short that accompanies the rapidly approaching Pokémon movie that premieres Saturday July 19th!

Highlights of Extremespeed Genesect: The Awakening of Mewtwo and Pikachu and its Eievui Friends
The hit movies that premiered last summer, "Extremespeed Genesect: The Awakening of Mewtwo" and its co-feature short "Pikachu and its Eievui Friends" are already going to be broadcast on TV for the first time!


Diancie, Princess of the Ore Country
"Diancie, Princess of the Ore Country" is tale about what happened to the mythical Pokémon Diancie, which plays a major role in the newest movie "Pokémon the Movie XY: The Cocoon of Destruction and Diancie", before she met Satoshi and friends.
Story
Diancie, the princess of the Diamond Ore Country, and her Melecie butlers set off on a journey to search for the legendary Pokémon Xerneas in order to save the Ore Country from ruin. But something gets in the way of their journey! What is it!?


Also: We will show 5 minutes of this year's feature film!


Broadcast information:
The special will air nationwide on all 6 channels in the TV Tokyo Network
(TV Tokyo/TV Osaka/TV Aichi/TV Hokkaido/TV Setouchi/TVQ Kyushu Hoso)
Go here for channel listings

Source: http://www.pokemon-movie.jp/topics/anime_2H.php