Just know how to control your bananas. If you're really good at control, your partner wont be in a situation much where he'll be slipping on your bananas constantly. It will happen every now and then, but it should be a rare occurance
you can teach your partner to glide toss, if it will help. Otherwise, just teach them to be able to avoid them on the ground and stuff, which really isnt that hard to do if they arent being thrown at you.
With diddy, be prepared to save your teammate alot. He's one of the faster chars in the game, and its a good chance he'll be the fastest on your team, since alot of the easy killers dont move that quickly (minus MK). So just pay attention to him and save him, when you can. They're gonna do some stuff that you just cant save them from, but at least recognize where it something that you can do something about.
When playing diddy in teams, you're most likely gonna be the stock tank. As in, you're racking up damage but you're doing it safely, and you always make sure you have the attention of one person from the other team. When you get their attention, do everything safely and try take as little damage as possible, but make sure you do enough so that he doesnt go over and attack your teammate. Sometimes its hard to play safe and rack up damage, so at least do your best to not put yourself in a situation to get easily gimped.
There are rarely any vids of diddy in teams, except mine. So you can look at my vid thread and look at the most recent teams matches of examples of what diddy does in teams. Im not perfect, but its better having some sort of visual evidence. Heres a link, if you care to watch:
http://smashboards.com/showthread.php?t=193194