var aa = document.getElementById("aa");
var bb = document.getElementById("bb");
if(aa!=null&&bb!=null)
{
    var aa_h=aa.clientHeight;
    var bb_h=bb.clientHeight;
    if(bb_h>aa_h)
        aa.style.height=bb_h+"px";
    else
    {
        bb.style.height=aa_h+"px";
    }
}