Ruling 2 Active Pokemon? *Edited with second question*

[Jahikoi]

I've noticed quite a lot of cards that say 20 damage to each of the defending pokemon. Does this mean there is actually ruling regarding matches with two active pokemon? I assume this idea was brought out during the same time Ruby and Sapphire introduced double battles into the gameboy games...

Like, Dark Dragonite

Dark Dragonite (#15, Team Rocket Returns)
Stage: Stage 2 HP: 120 Type: Darkness Weakness: C Resistance: FG
Power: Dark Trance - As often as you like during your turn (before your attack), you may move a D Energy card attached to 1 of your Pokemon to another of your Pokemon. This power can't be used if Dark Dragonite is affected by a Special Condition. (Poke-POWER)
Attack: [LW] Double Wing Attack - Does 30 damage to each Defending Pokemon.
Attack: [3] Claw Swipe (50)
Retreat Cost: 2 Rarity: Rare
Artist: Nakaoka

Edit:

I guess the real question I meant to ask, is how the heck do you play double battles? Because I have never seen anyone mention double battles, nor play one, nor specify any specific rules. Is there somewhere which has a rule list?

 

[King Arceus]

RE: 2 Active Pokemon?

If an attack states it does X to each Defending Pokemon, then it does X to both of your opponent's Active Pokemon in a double battle. Some effects will say to choose one defending Pokemon, this means in singles it always is their active Pokemon, but in doubles you have to choose which one. Hope I cleared it up some.~Mark

 

[DNA]

Double Battles are played like Single Battles; you either play with two players (in which you have 2 active and a maximum of 4 bench) or four battles (Multi Battles; each of you has 1 Defending Pokemon and neither can have more than 2 Benched Pokemon).

The rules are a bit more complicated (I forget where to find them), but mostly they are the same. Usually it has to do with the turn order that's the confusing bit.