I think the most important thing in this team is just understanding the marth's play style. Know when to shield so you don't get hit by his fsmash. Know when to get out of the way so you don't get comboed. And learn when to jump in and help him out.
both sheik's and marth's bthrow and fthrow lead into smashes (marth's dthrow works as well since it sends the opponent backward). As mentioned, marth's uthrow sets up for your aerials, but make sure that the marth player isn't trying to combo on his own as you'll get hit as well or you'll just be in his way (and as a marth player, i usually hate that in teams).
a very important thing to note is that you want to stay out of the marth's way when he's on the offensive. Marth likes to have some room in teams, and sheik doesn't like to get caught up in a friendly marth's combos. However, you also have to learn to use your speed to quickly save your partner whenever he's getting comboed by sheiks, falcos, foxes aerials, falcons, other marths, etc.
although marth may have the best edgeguarding game, sheik is still very good at edgeguarding. So really, the player that edgeguards should primarily depend on whatever is most convenient at the moment. You simply need to work this out with your teammate and get a feel for when each player does the edgeguarding.
watch ken and isai's team matches for a good idea of some more possibilities.
