Javascript Copy Text Boxes.

I know this is probably a real simple one, but I'm obviously missing
something..

I'm building a function that I'll use throughout a website in the situation
that I have two text boxes - the two text boxes will generally contain the
same data. After the user completes the value of the first textbox, I want
to onChange the value of the first textbox into the second textbox UNLESS
the second textbox already has a value.

Here's what I've done so far...

IN THE HEAD
function CopyTextBoxes(TextBox1,TextBox2) {
if(TextBox2.value = '') {
TextBox2.value = TextBox1.value}
}

IN THE BODY WITHIN THE FORM
<input name="MailFrom" type="text" id="MailFrom" size="60" maxlength="100"
onChange="CopyTextBoxes('MailReplyTo','MailFrom')">
<input name="MailReplyTo" type="text" id="MailFrom" size="60"
maxlength="100">