Agreed, there is 'something' there along with the riff but I think it's a moot point. Until Chuck explains how he played it, we'll never know. I personally won't put a lot of time into it especially when I rarely, if ever, play the riff.Here's a slower version of the riff looped three times. You can clearly hear that the low bass plays on during the high riff. While it's possible to do as a double stop, it doesn't sound like it on the recording. If that's not dubbed it's a guitar.
BTW, the low A goes together with the high F#, not the G as I assumed in an earlier post.