For this you could simply use frames.. Frame one page at 50%, left align, and another page at 50%, right align.. However, I don't really recommend it... Might be very awkward..
a better question than how is why. What reason do you have for desiring this solution - what are you hoping this will accomlish? Frames, although useful, bear the mark of tacky overuse and general disgust by most designers and artists.
Use a table to split the page into 2, then use php or another language to call the page into each part of the table.
This allows me to have a web page within a web page. insert this: <iframe src="secondpage.htm" name="NewsFrame" width="397" height="125" align="default" frameborder="0"> <p> </p> </iframe> Then name your second page (secondpage.htm) then change the width and height to better fit your scenario. you will still have to do some scrolling. hope this helps.