Hmmm, I was reading through the Owners Manual and it doesn't do what I want (which I'd expected)... I cannot play two amps to separate two cabs simultaneously in 'stereo'. Also there's no impedance adaptation, meaning the switcher won't adapt when switching between an amp set to one impedance (for ex: 16 ohms) to a cab of a different impedance (for ex: 8 ohms).
I think I'll just stick with the Ampete 444 and 88S / 88S Studio switchers. At least these allow you to play to up to two cabs from one amp, has a separate FX send/return feed per amp, and includes impedance adaptation.
http://ampete.com/products/switcher-series/