I haven't really seen a Marth-Puff team at high level but I think it could be a very strong team.
Regarding teams theorycrafting, doubles is all about creating a win condition out of your current situation. You always want to be the team with two players versus the one. Having the right strategy can greatly improve your chances. Think about how you'll want to approach fastfallers and floaties. Marth-puff will do well vs exclusive floaties, but a good fastfaller will rock your world if you aren't ready.
Marth is generally a support character in teams. If he gets too slap-happy, he'll get tossed around and burn stocks which is no good. The marth should be good at zoning people out, keeping stage control, spotting opportunities, and disrupting opponent pressure. If he does this well, he can stock tank well.
On the other hand, Puff is an all-rounder. She can play support or be a WMD. I suggest fishing for rest setups and testing how good the other team is at avoiding them. If your neutral and awareness is really good, you can weave setups into the chaos. I also suggest throwing people offstage and edgeguarding.
As a team, grab combos should be deadly. Marth's stupid grab + puff's stupid rest = stupid good team. You want to stay close to each other so you can follow up, and be creating opportunities to grab. Marth can also fsmash fastfallers offstage for you to edgeguard. Both marth and puff are good at zoning and can avoid committing to stuff if they so choose. Use this to find opportunities out of the corner or when corner pressuring others. If your marth likes to throw out fsmashes a lot, be ready to shield or stay out of his zone. Otherwise the two of you shouldn't really get in each others way like most other teams do.
As far as practice goes, practice resting out of marth's grab. That's the main thing. If you wanna go above and beyond, practice resting different DI's out of marths fthrow and downthrow.
Practice zoning just by playing each other, and get used to thinking about stage control. You can discuss like the kind of ranges marth and puff can individually cover, and how to best utilize that on different stages. That's getting a little abstract though. The bread and butter team rest setups are really what you want to get down before tournament. Once that's solid, you really just want to practice playing different teams. Being aware of what's happening to both you and your teammate is challenging at first.
