Neko, trust us, the current DD ICs have is terrible for Nana as it doesn't work like it should. She has the same window as her vBrawl self so really, it limits her rather than helps desync them. While Popo DDs over to an opponent, Nana will be lagging behind from the turn around animation, which wouldn't be very good.
Fixing the current DD for them will help, it's not that they won't be able to desync anymore from it, as they likely will be able to still. It's just that in its current state it is VERY bad.
I mean, what do I know. I dont use ICs seriously. Its just that their current DD lets me do all sorts of desyncing. I'd be better at it if I actually played ICs. xD