I think you might've misread GP. (Or maybe I did?)
He's not saying you would use it as two separate 120v circuits sharing a ground but rather as a single 240v circuit. His point is that it's easy to rewire for 240v since it's the same as all the other wiring in your house just with both poles exposed.
Of course you do have to run a new wire rather than repurpose what's already in the wall since you need the entire circuit to yourself. So I think it's not as trivial as he's making out.
But then at that wattage you'll also want to punch an exhaust fan in for waste heat so it's not like you won't already be making some modifications.
The wiring (at least in the US) to the 120V outlets is just one half of the split-phase 240V. If you want to send 240V down a particular wire, you can do that, by changing the breaker, but then you lose the neutral. You also make the wires dangerous to people who don't realize that the white wire is now energized at 120V over ground. (Though it's best to test to be sure anyway, as polarity gets reversed by accident, etc.) Live wires should be black or red.