jQuery Forum

One way to do it is with a jQuery carousel. They are normally used to display multiple images on a small section of the page, but a carousel could also display 2 text selections on a large section of the page and slide them back and forth.

A lot of free plug-ins are available. Do some Google searching.

Here is 1 example that scrolls 3 text-like selections across a variably sized window.  Use your hand to block part of the window, and I think you get the behavior you are looking for. You get the sense the plug-in has lots of flexibility, and the creator says it has a configuration robot.  I have not used it, but it seems close to the behavior you describe.

http://caroufredsel.dev7studios.com/examples/variable-visible-variable-size.php