Try the following,
<html xmlns="http://www.w3.org/1999/xhtml">
<head runat="server">
<title>CheckBox Toggle</title>
<script type="text/javascript">
function aspcheck1True()
{
document.getElementById("CheckBox2").checked = false;
}
function aspcheck2True()
{
document.getElementById("CheckBox1").checked = false;
}
</script>
</head>
<body>
<form id="form1" runat="server">
<div>
<asp:CheckBox ID="CheckBox1" runat="server" onclick="aspcheck1True();" />
<asp:CheckBox ID="CheckBox2" runat="server" onclick="aspcheck2True();" />
</div>
</form>
</body>
</html>
Watch the live action,