Hi
I'm not sure that you can, but you can use snap to position the windows in only a couple of seconds.
Just grab one window near the right side and drag it until the cursor hits the right side of the screen.
It will format to use the right half of the screen.
Not grab the second window and do the same thing on the left side and you will have the two windows formatted to fill the screen side by side.
You can literally do this in 2 seconds.
If you drag the windows up until the cursor hits the top of the screen it will instantly format to full screen.
This was my favorite thing that was added in Windows 7.
I use it constantly all day long, because I spend a lot of time with 2 open applications.
My browser and Word, so I can drag...