How would I make these two divs be the same height, no matter the length of either div? I've tried giving the divs a container div and setting the div height to 100%, but it doesn't seem to work.
You can't. Not if the content needs to be able to expand or you don't know the height of either of the divs.
You can make it appear like they have the same height. Using faux-columns. Wrap the divs in a containing div and set a repeating background image (1px high, as wide as the containing div) in it, with a border, or different background colors.