i don't like using nairs vs ic when they are together. they can cc it or really lag it with their double shield. until the nair hits through both of them and you land it takes too much time.
instead, you gotta carefully approach with bairs (perferbly csticked) and dairs mostly, and throw in a suprise forward b once in a while. it kinda owns them, as it separates them and often nana will 'cancel' your lag. land behind them is what you should usually do.
dont be tempt to grab them when they're together unless you are sure its popo you are going to grab.
if they start dancing with their desynch stuff, i like backing off and take my time to find a valnurable spot.
once they are separated, try and knee nana away first unless popo is really close. dont screw around with falcon punches or fsmashes lol, nana can really own you with a jab or just run away. if popo is close be very caution if you are going to attack nana.
a single ic is easy. he combos so good out of a dthrow and its easy to start and edgeguard infinite (you bair , he does forward b, repeat 6 more times till popo is dead)
by the way, dont get grabbed. if you do get grabbed, know that unless he does a short hop dair with nana you can easly escape with di away\times double jump. but watch out not to di when he is gonna fsmash you out of the grab.