That is pretty cool! It works on my old PRS505 as well as my PRS-T1, and on my BeBook-2010. It's not flummoxed by changing font sizes, too.
But I'd hate to have to do a long document by hand... I think I'd want to experiment with interleaving by line or paragraph, as well. A lot of Canadian legal docs are published in two columns, aligned by paragraph, French on one side, english on the other. Perhaps a wide right margin for one, and a wide left margin for the other would look OK.
Ideas are free... it's the implementation that costs money.