kom på:t
function setBoxValue(Box1, Box2)
{
var value = Box1.options[Box1.selectedIndex].value;
if(Box2.options[Box2.selectedIndex].value =='')
{
for(var n=0; n<Box2.length; n++)
{
if(Box2[n].value==value)
{
Box2[n].selected=true;
return null;
}
}
}
}
<select name="Box1" onChange="setBoxValue(this, this.form.Box2);">
<option value="123">Första</option>
<option value="456">Andra</option>
</select>
<select name="Box2">
<option value="123">Första</option>
<option value="456">Andra</option>
</select>