Double ICs is terrible. I've done it before for fun, but I would never do it in tournament. The only stages it really works on are dreamland... and maybe fd or stadium... you need a lot of room...Basically, ICs in doubles control the ground while their partner plays mostly on platforms.
Characters that work well for this are Fox, Falco, Sheik, Falcon, and (sometimes) Marth. You can also do puff or peach with some success, but I'm personally not a fan of either of them as partners, but I prefer puff to peach.
Once you get out of the neutral position, you have two roles:
a) You are a wall. Your partner beats the hell out of one opponent and you prevent the other from approaching/helping them
b) You combo one opponent to death while your partner acts as the wall.
In neutral you're pretty much gonna shoot a lot of iceblocks, blizzard a lot, and use a lot of dsmash.
ICs actually tend to wrack up the majority of the kills in doubles, and they can stock tank as well as long as your partner knows to keep opponents away from the ledge while you're recovering.
You have to develop a good sopo to do well in teams, because your nana is going to die very quickly a lot of the time and you'll just lose her in the chaos of doubles, but if you have a strong sopo, it doesn't matter too much, because in doubles it's a lot more fundamentals and spacing/positioning.
if you have any more specific questions, I can clarify for you.