codice:
<SCRIPT>var ok=false</SCRIPT>
<FORM METHOD=POST ACTION="">
<input type="radio" name="uno" onclick="ok=true">
<input type="radio" name="uno" onclick="ok=true">


<input type="radio" name="due" onclick="if(!ok){alert('Seleziona uno');this.checked=false}">
<input type="radio" name="due" onclick="if(!ok){alert('Seleziona uno');this.checked=false}">
<input type="radio" name="due" onclick="if(!ok){alert('Seleziona uno');this.checked=false}">
<input type="radio" name="due" onclick="if(!ok){alert('Seleziona uno');this.checked=false}">
</FORM>
ciao