I'm not a big doubles person myself, I prefer singles instead. Doubles is still fun though, lol.
In doubles I like to play a very aggresive style especially because theere is less opportunites to get punish for mistakes with a partner because it makes it harder to capitlize mistakes, when they have to worry about 2 characters.
I usually teamed with a Marth and he'd stay back most of the time and play more cautiously, while I went to rack damage.
Usually when aggro in doubles, d-air works wonders its very quick and has good priority so if the opposing teammate goes to jump in he might get caught. Plus they get popped up in the air which can lead to a string if your opponent is ready. Fireballs are also very good, if you set up a wall where your behind your opponent spamming Fireball's. Depending on which characters you're playing against they'll be forced to play a more cautious style, and take damage while trying to get through the wall. Not to mention, Mario has decent at racking damage in my opinion, and teaming with someone who can score KO's fairly efficiently would make a good team.
I found Luigi is a really good partner (as many have said), I teamed with 1 a while back for the first time and we played pretty decently. But I mean it depends on how you and your opponent play's, and how well you guys work together as a team, communication is probably key too in doubles.
I've never played against Double MK, but you're safest option is to play more cautiously, because luckily Metaknight doesn't have a projectile, and your Fireball goes through all of his moves. It also makes it more harder for him to Tornado because both you and your opponent can capitilize on punish it depending where they're Tornado'ing from. Mario doesn't get too badly owned by the Tornado, but if you're teaming with a character who get's hurt by Tornado, it could cause some issues.
Also experiment with your parenter/character try to set-up some good opportunites to use Cape to score some very early K.O's. Grab's are probably your most reliable way of doing this. Especially some up-throws (Jigglypuff's u-throw has huge knockback, imagine with Cape O_o.)
Sorry for wall of text, and I hope some of this information proved useful.
