Yeah, the first hit is a set-knockback blow that is intended to lead them into the second blow... but a number of elements (standing on a slope, DI or just lucky positioning) can cause it to miss -- against characters with good (or even really just not bad
) recoveries, the generally isn't enough to get the kill (or even set up the gimp), but it certainly gives you an advantage -- they're off the edge a ways, and if you're comfortable with attacking them out there (which you should be -- you're Toon Link for crying out loud) you can make it lead to a kill (or, if you're ballsy, a spike... more likely to just turn into an SD, though
).
Now, there's also another scenario that I've experienced many times before -- both hits landing, but still causing horizontal (backwards, specifically) knockback. I've seen this directly kill where the set knockback of the first hit almost certainly wouldn't, but I'm pretty sure it's really just lucky situations that are out of the TL player's control (realistically). It may have been smash DI on the second hit, or some sort of lucky spot on the hitbox, but it's something that I'm just thankful for when it happens. --EDIT: For the record, I'm talking about higher-percentage scenarios where this happens... I know about the whole "below ~20%" situation, but a similar thing's happened at higher percentages, and actually *added* to the fixed knockback (gotten side-kills in places where normally it would not... and not due to obvious failure in DI).
If you want to work on hitting with the first-only, definitely check out the links vbdood posted, they cover it pretty well. It's never really been a great kill move to me, seeing as how there aren't many Ike's in tourney, but it's still a good thing to know about.
--UnderWing