Hi Shona,
Something along these lines.
<script type="text/javascript">
function checkBoxes(f){
if(f.drop1.selectedIndex ==0 && f.drop2.selectedindex==0){
alert("Your Error Message");
return false;}
return true;
}
</script>
<form onsubmit="return checkBoxes(this)">
<select name="drop1">
<option value="">Choose Something</option>
<option value="option1">Option 1</option>
etc
</select>
<select name="drop2">
<option value="">Choose Something</option>
<option value="option1">Option 1</option>
etc
</select>