Pokemon double battles in Diamond and Pearl are more annoying than in past games.
The main problem is that there are moves that effected both opponents and your partner Pokemon. For example the move Surf attacks all Pokemon in the battle.
So how do you create successfully pokemon partners for battles?
Very simple really.
We suggest you try a rock type with a electric type. EG. Geodude and Picharisu. We choose this because electric moves have no impact on rock type. Further more Picharisu has the move Discharge and Swift that effect both you opponents.
We also suggest you try a rock type with a flying type. EG. Starly and Geodude because when we teach Geodude Earthquake or magnitude – both killer moves for anything that does fly or levitate – so Starly remains unaffected.
These are basic groupings but a good start to making effective double battle teams.