Well, Bright Eyes released an acoutic-y record and an electronic-y record at the same time. Not as wildly divergent as mariachi and punk, but sort of sounds like a similar schtick.

And, yes, it is schtick, even if both records are pretty good.

Opeth. Traditionally a death metal band, they released Deliverance and Damnation nearly simultaneously in 2002. Deliverance was their heaviest album to date, but Damnation was a throwback to 70s progressive rock, with no metal tendencies at all. This wasn't completely unprecedented, as Opeth's normal style is filled with light/dark contrasts, with soft, acoustic passages interspersed within and between their brutal, heavy songs; but they'd never made an entire album dedicated to the "light".

I believe that Cypress Hill once simultaneously released 'rock' and 'rap' albums. I'm not sure, though--when I heard this information, I was probably high.