iCloud always synced emails since it was introduced in iOS 6. If I read an iCloud email on my iPad, the notification and badge goes away on my iPhone and vice versa. I wish it did that for Yahoo Mail, but it doesn't.
As far as I can tell notification sync isn't working. None of my app's notifications on my iPhone clear when I clear them on iPad or vice versa. The only ones that do are ones that always did like the Gmail app.