Update your function
Load(url1, url2)
.
On your clientclick code, try this
onclientclick="return Load(this.value,document.button2.value);"
Your client click event should work.
OR
do something like this.
button 1
<br />
onclientclick="return Load('<%=button1.ClientID%>','<%=button2.ClientID%>');"<br />
then change your Load function to
function Load(btn1, btn2) {
var btn1Value = document.getElementById(btn1).value;
var btn2Value = document.getElementById(btn2).value;
}
Good luck!