No matter how long I've been doing it, I'm constantly learning new things. I know that double or quad tracking guitars for a bigger sound is common in metal. So, I've been fooling around with it a bit this past year for my own stuff and haven't been terribly happy with the results. Now that I have the Kemper, the primary reason for my dissatisfaction is actually even more prominent - the chorusing / comb filtering and other phase artifacts you get.
It's worth noting that I don't write or play metal, but rather gravitate towards more classic rock type stuff. Think full or 4 note chords versus the common stylistic approach of single or dual note riffs or grooves in metal. What might sound fuller on a two note riff of moving fifths can quickly sound like a Boss Chorus has been added when playing chord based material.
Prior to the Kemper my miking would vary. Sometimes a 57, sometimes 57/421, occasionally an additional room mic at either a foot or at distance, etc. I usually tried to work with two tracks on the rhythm part, so a 57/421 (with phase alignment checked) would count for one track. The comb filtering is most noticeable when both parts are at the same pan position, and you can get away with it the most if you pan them hard L/R. Which is great until you push the Mono button to check the mix.
I've also tried playing the part twice (which I can keep fairly tight) as well as tracking DI and then reamping into two different amps or settings. I've done this on the Kemper as well, reamping into two different profiles with a similar vibe. Typically we're talking high gain stuff here, albeit the classic rock variety. Think Eddie's brown sound versus the incredible saturated tones of modern metal. Perhaps it's because my miked amps were typically mono tracks and my Kemper tracks are stereo, but this comb filtering effect is even more noticeable with the Kemper. And it wasn't subtle in miked amps to begin with.
There was a great thread from back in July that talked a lot about multi tracking in general. Fun stuff, but surprisingly there was very little mention of the problem I'm talking about here.
Double tracking vs quad vs more? Is it only for sloppy players?
Thus far, my conclusion is that while this technique can give you fuller tracks for the single / dual note riffs of metal, it may not be suited for more Van Halen / Bon Jovi - ish styles that have full or mostly full chords featured prominently. @Cederick's original examples are the single / dual note style (and the guy's a freakin' precision machine on guitar), so there doesn't seem to be the opportunity for the chorus pedal effect.
I would be interested in whether or not this technique has worked well for those of you who play more chord based rock. It could be that I'm missing something fundamental in how I apply this technique, and it's equally possible that square pegs are simply not meant for round holes.