//get the height of the box 1a
var box1a_height=document.getElementById("home_box1a").offsetHeight;
var box1b_height=document.getElementById("home_box1b").offsetHeight;

if (box1a_height > box1b_height) {
    document.getElementById("home_box1b").style.height = (box1a_height-43) + "px";
} else {
    document.getElementById("home_box1a").style.height = (box1b_height-43) + "px";
}