True Double Battles are an element only seen before in Pokémon Games in Link battles have been added to Pokémon Emerald. This is where you battle with a Partner against another 2 people

At the start of battle, you get asked to pick 2 or 3 of your Pokémon to use in battle, in the case of Battle Tower, 2. The Top is your Pokémon and the bottom is the computer trainer's Pokémon
In it you only have control of your Own Pokémon. The other person on your team, controlled by the Computer, controls it's own Pokémon and you have to work with your Computer partner and win. All the Speed Stats are determined like normal so it's just a normal battle but with a team mate

There are only a few places that you are capable of battles like this and here is a list of them

First one you'll see is in the Space Center battling Team Magma Executive Tabitha & Team Magma Leader Maxie. Steven will come and help you and will battle with you with Metang and his other Pokémon
The rest are all in the Battle Tower. One of the battle options is a True Double Battle where you pick your team mate and go to Battle to see how high of a streak you can get to.