Looks like he's got the rhythm guitar ducking the kick drum and then the result of that ducking the vocal. I guess it makes most sense if you want drastically different settings for the two processes. Like, you probably want a pretty quick attack and release on the one ducking the kick drum so that it actually gets out of the way of those transients and then comes back without too much lag, but then you want it to respond to the vocal a little more smoothly with a bit longer time constants. Or maybe you really want it to dip way down when the kick hits so you set the ratio on that one really high but you want just subtle ducking from the vocals, so that ratio is set low. Or whatever. I'm not trying to speak for Steve, but it's pretty obvious to me how the two techniques could be noticeably different.
Edit - and again, this could be done using two tracks, but it's another situation where I would just as soon keep it on one to cut down on clutter, avoid pan law whackiness, etc.