Replying to Talib Kweli has two voices in his songs...?

So I was listening to the song "Beautiful" by Talkb Kweli, which I've had on my iPod for a couple years and have heard too many times, and it definitely has two different voices that switch every couple of seconds. Anyone else ever notice this or is this well known? And what's the deal with it? Just realized that I wasn't just too high all those times I thought his songs had both him and Mos Def in them. +K for someone linking the YouTube link for the song, I'm on my phone on my way to class.