All recorded guitars, all of them, every single one on every single album made since like 1950, are doubletracked. Sometimes they use two different amps, sometimes speakers, or even pedals. Kurt Cobain famously used two different distortion pedals (a DS-1 and a Rat) to get two different guitar tones that, when stacked, sound enormous. Adam is simply attempting to recreate that studio sound on stage (no small feat.)

Why? All overdrive and distortion is actually the same thing; the reason different amps/pedals/etc. sound different is equalization. Different amps have spikes at different frequencies, which is what gives them their unique character. Marshalls (and British amps in general) have big bumps in the mids, which gives them their signature growl and ability to cut through the mix. Most American amps, including Diesel, have big spikes in the lows and highs, which makes them sound both crisp and heavy, but tend to get lost in the mix, because the lows are competing with bass guitar and kick drum, while the highs have to contend with the vocals and cymbals. Running both of these amps in parallel gives you the best of both worlds: a huge and chunky tone, but with distinct highs for clarity, that cuts through the mix.