If you look for route 1, 3, 5, 6, 7, 9, 10, 12, 15, 16, 18, 20, or 23 you will find some dark grass for double battles.
But keep in mind that route 9 will be frozen in bw2 until you get through the main game.
There's also the forests pinwheel forest and lostlorn forest.
Virbank Complex, Castelia City, Abundant Shrine, dreamyard, Cold Storage, Dragonspiral Tower, and Victory Road.
Some dark grass doesn't always mean double battles. But these are all the places I could think of that have dark patches of grass in them.
Hope this helps.