Here is some JavaScript code to toggle check boxes (same action as radio buttons).

In the page load event register the JavaScript: 

   chkTwoYear.Attributes.Add(“onclick”, “javascript: CheckTwoProgram(” + chkTwoYear.ClientID + “);”); 
   chkFourYear.Attributes.Add(“onclick”, “javascript: CheckFourProgram(” + chkFourYear.ClientID + “);”);

In the HTML of the page add the JavaScript:

<SCRIPT Language=”JavaScript”><!–
function CheckTwoProgram(id) { 
   if(id.checked) 
      document.forms[0].chkFourYear.checked=false; 
   }
function CheckFourProgram(id) { 
   if(id.checked) 
      document.forms[0].chkTwoYear.checked=false; 
   }
//–>
</SCRIPT>