<asp:RadioButton ID="selectAddress1" runat="server" AutoPostBack="True"
Checked="true" OnCheckedChanged="checking_validation_first" /></b>

<b><asp:RadioButton ID="selectAddress2" AutoPostBack="True"
runat="server" OnCheckedChanged="checking_validation_second" /></b>

on code behind page -

protected void checking_validation_first(object source, EventArgs e)
{
if (selectAddress2.Checked == true)
{ selectAddress2.Checked = false; }
}
protected void checking_validation_second(object source, EventArgs e)
{
if (selectAddress1.Checked == true)
{ selectAddress1.Checked = false; }
}