Marcar somente 2 CheckBoxes

<script language="javascript">
total = 0;
function Conta(objeto)
{      
if (objeto.checked)
        total++
else
        total--

if (total >2)  
{
        objeto.checked = false;
        total--
}
}
</script>

<form name="frm">
<input type="checkbox" name="chk1" onclick="Conta(this)">
<input type="checkbox" name="chk2" onclick="Conta(this)">
<input type="checkbox" name="chk3" onclick="Conta(this)">
<input type="checkbox" name="chk4" onclick="Conta(this)">
</form>