try this
<asp:Button runat="server" ID="btnSave" Text="Save" OnClientClick="return Validate();" OnClick="btnSave_Click" />
function Validate()
{
var ddl1 = document.getElementById('<%= ddl1.ClientID %>');
var ddl2 = document.getElementById('<%= ddl2.ClientID %>');
debugger;
if (ddl1.options[ddl1.selectedIndex].text == '' || ddl1.options[ddl1.selectedIndex].text == '')
{
alert('Please enter the data');
return false;
}
return true;
}