The Eternal Rivals shall emerge victorious with mid/high difficulty IMO.
Don't forget that 2 targets is harder than 1, and if one of them got caught on Genjutsu, the other one will help him get out of it, and I don't think Itachi will have time to cast a Genjutsu because while he's trying to catch one of them with it the other one won't be standing by and watching, that's that..
And Gai alone defeated Kisame by himself, how about him AND Kakashi against Itachi? People are overrating Itachi by thinking he can win against those two, just because he have Susanoo/Amaterasu/Genjutsu doesn't mean he can win, remember that Susanoo won't stay long, they can keep up with him until his chakra is drained by overusing it and catch him off-gourd..
Anyway, that's just my opinion. Hope it didn't offend anyone.
Peace