Answer by Just code for Set height of = to height of another through .css

I am assuming that you have used height attribute at both so i am comparing it with a heightleft do it with JavaScript.

var right=document.getElementById('rightdiv').style.height;var left=document.getElementById('leftdiv').style.height;if(left>right){    document.getElementById('rightdiv').style.height=left;}else{    document.getElementById('leftdiv').style.height=right;}

Another idea can be found hereHTML/CSS: Making two floating divs the same height.