Okay, here we go.
Add your radio question (do NOT use the "Always hide this question" setting) and then add the following script to the source of the radio question text or help. Replace "MM" (line 6) with the ID of your multi-choice question and "HH" (line 7) with the radio ID.
The script basically does what you described above - hides the hidden question and then interrupts the Next/Submit function and toggles the hidden radio depending on the state of the first checkbox.
<script type="text/javascript" charset="utf-8">
$(document).ready(function() {
// The question IDs
var qMultiID = MM;
var qHiddenID = HH;
// Hide the hidden question
$('#question'+hiddenID+'').hide();
// Interrupt next/submit and toggle the hidden question depending on the state of the first checkbox
$('form#limesurvey').submit(function(){
if ($('#question'+qMultiID+' input.checkbox:eq(0)').attr('checked') == true) {
$('#question'+qHiddenID+' input.radio:eq(0)').attr('checked', true);
}
else {
$('#question'+qHiddenID+' input.radio:eq(1)').attr('checked', true);
}
return true;
});
});
</script>
If you want to test it without hiding the question, comment out this line:
$('#question'+hiddenID+'').hide();
Like so:
//$('#question'+hiddenID+'').hide();
