I can see this being a worthy team. I actually think it's one of those teams where either one can be approaching the other team and the other one camps. It's like a switchoff thing. We can combo aerials with each other, from Luigi to back airing/forward airing into our back air/forward air, or vice-versa. Not only can King Dedede down throw into Luigi's upB, if the situation seems different where Luigi grabs, he can just throw into one of our aerials easily, since his throws are so basic and it's easy to get used to the trajectory of their knockback. Luigi can also down throw into an appropriate aerial into one of ours. You can forward throw into our forward smash for a double-wammy which is an automatic kill as accompanied to my down throw into your upB. A very combotastic team. King Dedede is always the stock tank, so he can stay behind and let Luigi show off his typical metagame on the other side. I think Luigi can backthrow while the spinning body can hit nearby characters, so when he's up there he can backthrow when King dedede sees it without worrying about being punished by the teammate of the other team too.
Additionally to what I said about either one approaching, the killers that you're trying to look for can be either Luigi or King Dedede because we both have great kill moves anyways. Your forward smash also, **** forgot lol. It can just be on-the-spot for this team on who kills since both characters can kill nicely in doubles.
King Dedede would be the safer gimper because of his back air and his upB's super armor to get back to stage without being gimped himself. Luigi should stay on stage definitely. And I gues we can have a couple moments where we have projectile fests. Luigi shorthops fireballs and we throw waddle dees from below, and possibly a Gordo.
Our best stages would be the following: Rainbow Cruise because King Dedede does excellent here with just about any partner, and I normally beat the opposing team on Rainbow Cruise even if they have Meta Knight because we can kill so early. Luigi's recovery options are astounding so he has no problem being on the stage. There's no stage advantages to team tactics but it's just a great stage overall for this team. We'll do well on any neutral, no worries there. That's where I think up of all of what I'm saying for this team anyway, the neutrals. This team is excellent now that I'm thinking and writing about it, I'm surprised I didn't think of Luigi before...