I've got dual screens on my home desktop (not the one in my apartment). Haven't ever bothered trying to put blockland on dual screens. It probably wont work as the game doesn't have a resolution for a [very] widescreen.
Someone could probably make a mod to be able to have a custom format though, but it'd distort the field of view.
I usually use the two screens to have a photo of what I'm building on one of them.
I have no sources from my guesses, so they could be wrong.