the main thing in doubles is that your opponent gets to choose who to attack making walling and luring attacks to switch into harder and more difficult to predict. you need to find team members that cover each other both with moves and abilities. here is an example of my Battle Subway Doubles Team:
Hippowdon @ Rocky Helmet
Trait: Sand Stream
Impish Nature (+Def, -SAtk)
EVs: 252 Def/ 212 HP/ 44 Atk
- Earthquake
- Yawn
- Stealth Rock
- Slack Off
Sets up Sand to help stall and break Sashes. Being in-game, it is much easier to out-stall the computer than a real opponent.
Stoutland @ Choice Band
Trait: Sand Rush
Adamant Nature (+Atk, -SAtk)
EVs: 252 Atk/ 252 Spd/ 4 SDef
- Return
- Superpower
- Wild Charge
- Crunch
With Sand Rush and Hippowdon as a lead duo, Stoutland automatically begins with x2 Spd and x1.5 Atk, and is immune to Sand damage. it can easily devastated while Hippowdon sets up Rocks and Yawn Spams.
the other half:
Heatran @ Leftovers
Trait: Flash Fire
Timid Nature (+Spd, -Atk)
EVs: 252 SAtk/ 212 Spd/ 44 HP
- Earth Power
- Torment
- Fire Blast
- Substitute
Sets up a Sub and uses Torment in anything that may be carrying Earthquake or the like. Then it can freely Fire Blast or Earth Power you every other turn and then put up another Sub, forcing you to focus both pokemon at Heatran or waste turns trying to break Subs until Heatran us too weak to make another.
Gastrodon @ Life Orb
Trait: Storm Drain
Modest Nature (+SAtk, -Atk)
EVs: 252 SAtk/ 144 HP/ 112 SDef
- Scald
- Stockpile
- Ice Beam
- Recover
this thing is beastly. Storm Drain stops the opponent from Threatening Heatran with Surf, andgives Gastrodon +1 SAtk. Stockpile adds to its already respectable bulk, and Recover keeps it alive. Life Orb plus STAB Scald is killer, and a Burned opponent can't touch Gastrodon after 3 Stockpiles. this covers Heatran perfectly, and i easily achieved a 49 Win Streak with my Sand Team.
so, i'm not saying you need to use this exact team, but if you look and see where each teammate covers the others, it will give you a better idea of the Synergy needed for Double Battles.
Hope this helps :)