Well, from watching the Newlyweds play a lot, and from teaming with PC's Marth somewhat often, there are a lot of great team combos Peach/Marth can pull, ESPECIALLY from each character's DOWN throw. If one of you gets a grab, make sure to combo off of it. If Peach is behind a Marth who has someone grabbed, make sure he dthrows. You can either dsmash (if they're a spacey, or other character at low %s) or charge usmash if they're at a high % for an almost guaranteed kill)
Same thing with Peach, her dthrow sets up for an easy tech chase (you're already covering one of the tech options, the Marth should be able to either combo from Peach's dthrow or cover the remaining 2 tech options, as well as no tech, as long as they have a fairly good reaction time/team mindset) This is all even better if you dthrow them and they're sent towards the ledge, in which case their tech options are limited even MORE.
Also, edge guarding... the Newlyweds will always have Wife edge hog, Husband edge guards as Wife rolls off the stage. It's ridiculously safe, and effective, and covers a ridiculous amount of options.
One of the most important parts of teams is being aware of what's going on, constantly. Having a great reaction time for when an opponent is launched over your way is fairly quintessential, too. Excellent team combos are what separate mediocre teams from the best teams.