I'm just recently becoming more accustomed to singles play but if there's anything I know it's doubles. King Dedede can be a serious threat in doubles if played correctly. In doubles there are usually two types of players: the dominant player, who goes for all the kills and likes to take people 1-on-1, and the support player, who distracts the other teammate while their partner is getting mercilessly edge guarded or follows their own teammate's movements in order to set up team combos. King Dedede is in my mind one of the top support doubles characters in the game. His grabs are easy set ups for team combos and also get your opponent's teammate out of the way when you want a chance to edge guard or team combo more effectively.
Here's some rules to live by:
1. You should always chaingrab in doubles if given the chance.
King Dedede's chaingrab is his best asset. Instead of not using it as much, adapt to the chaos doubles breeds and use it to your advantage. Chaingrab right into your teammate who can then hit them for you allowing you to follow up making a devastating team combo. Don't forget that the F Throw and B Throw are extremely fast and hit everyone in the radius of your hammer. If you are chaingrabbing someone and their teammate comes to save the day, throw their own partner at them. Chances are they'll get hit by the throw animation, their own teammate, or both.
2. D Throw is the best move in the game for setting up team combos.
If you didn't know before reading this, King Dedede's U Tilt has invincibility frames on it. This allows you to do amazing things with your partner to rack up damage as well as get early kills. All you need to do is D Throw, have your partner space enough away and time an attack that sends the opponent right back at you. When the opponent is shot back, use the U Tilt. If timed correctly you will have successfully chained three moves together, all powerful, racking up a ton of damage sending them sky high, usually to their deaths. My favorite set up is a D Throw into a Metaknight D Smash right back into an U Tilt of your own. It is not only cool looking but so fast that you can perform it regularly during your match! Try it for yourself!
3. You are big so stay out of giant cluster ****s or you will be eating a ton of damage.
King Dedede plays the support card really well. However, he can become a burden if up against a Metaknight who really likes to edge guard. Try to team with someone who can prevent you from getting mercilessly team comboed which King Dedede is subject to due to his large size. Teams at a high level is just organized chaos. Try to play the tank role and just stay alive throwing your opponents into fully charged smashes.
train your DI more and spacing dude it's key.