I have 3 divs:
If you set both div "one" and div "two" to have a height of 100%, you can control the height of both of them via the style of the outer div, and you are guaranteed that they always have the same height.
If you always want div "two" to be as high as div "one" or shorter, here's some JavaScript code that may help you:
document.getElementById("two").style.maxHeight = document.getElementById("one").style.height;