I've had solid success with this team for a while now. In my region I've placed 1st in doubles for 2+ years, and I recently went to a BC tournament a week ago and placed 1st without giving up a match. Though it could just be the chemistry with me and my partner for teaming so long... lol.
As for what Marth should do once you get a punish, well he can try and hit the flying opponent for some extra damage or go for a juggle if he's not too busy on his end. Otherwise Marth should just stick with his basic routine of keeping one or both guys on one side of the map and covering options with the occasional support when he gets a chance. Edguarding is pretty important obviously, as it keeps one guy off stage, racks up damage and forces his teammate to help which gives Snake opportunities on his end of the map.
Proper coordination from both Snake and Marth makes this a solid team imo.
some doubles matches to give you an example:
http://www.youtube.com/watch?v=nkXhvxk1TiM
http://www.youtube.com/watch?v=iQBGeWHYPVk