I discovered the following script here:

<script language="javascript" type="text/javascript">
function limitText(limitField, limitCount, limitNum) {
if (limitField.value.length > limitNum) {
limitField.value = limitField.value.substring(0, limitNum);
} else {
limitCount.value = limitNum - limitField.value.length;
}
}
</script>

This works great, but I need to check the length on two concatenated fields. Javascript rookie. Please advise