I know this is a very late reply, but this code is a tad more flexible and should help latecomers like myself.
function copycheck(from,to) {
//retrives variables "from" (original checkbox/element) and "to" (target checkbox) you declare when you call the function on the HTML.
if(document.getElementById(from).checked==true)
//checks status of "from" element. change to whatever validation you prefer.
{
document.getElementById(to).checked=true;
//if validation returns true, checks target checkbox
}
else
{
document.getElementById(to).checked=false;
//if validation returns true, unchecks target checkbox
}
}
HTML being something like
<input type="radio" name="bob" onclick="copycheck('from','to');" />
where "from" and "to" are the respective ids of the elements "from" wich you wish to copy "to". As is, it would work between checkboxes but you can enter any ID you wish and any condition you desire as long as "to" (being the checkbox to be manipulated) is correctly defined when sending the variables from the html event call.
Notice, as SpYk3HH said, target you want to use is an array by default. Using the "display element information" tool from the web developer toolbar will help you find the full id of the respective checkboxes.
Hope this helps.