OK,I will try to be as clear as possible.
I have one div (container) 500pix height containing two another divs in it. One of them is 200 height and another one that is on bottom is 300. Now if I manually change dimension of div that is on bottom to 200 pix, I want the one that is on top to get down as much as it can (so it's 100pix in this case) to the top of the one div that is on the bottom.
So if you understand me I want to make the position of the div that is on the top to depends on the height dimension of the div that is on the bottom, and to always drop down to it so they touch each other.