try this javascript function:-
function validate() {
if (document.getElementById("<%= ddlSlct.ClientID%> ").value == "1") {
if (document.getElementById("<%= taxbox1.ClientID%> ").value != "") {
alert("textbox1 is empty");
}
else {
if (document.getElementById("<%= taxbox2.ClientID%> ").value != "") {
alert("textbox2 is empty");
}
}
}
<asp:Button ID="btnSubmit" runat="server" OnClientClick="validate();" />
make changes in function as per your validation.
i have just check if it is empty or not.