Use javascript or server side event for it
see below snippet
<script labguage="javascript">
function enableDisable(objID)
{
if (objID == "btnLogin")
{
document.getElementById("btnLogin").disabled = true;
document.getElementById("btnLogout").disabled = false;
}
else
{
document.getElementById("btnLogin").disabled = false;
document.getElementById("btnLogout").disabled = true;
}
}
</script>
<asp:button runat="server" id="btnLogin" text="Login" onclientclick="return enableDisable(this.id)" />
<asp:button runat="server" id="btnLogout" text="Login" onclientclick="return enableDisable(this.id)" />
hope it helps