Chris,
You can attempt this JavaScript function. This will check to see if either value is filled. If both values are filled then it will concatenate them. If not it will leave the field blank. There are two function calls to trigger based on either field when changed.
$(document).ready(function(){
/*concatenate escalated user name*/
$('.escalateLName').on('blur', 'input', EscalatedApprover);
$('.escalateFName').on('blur', 'input', EscalatedApprover);
function EscalatedApprover(){
if(($('.escalateLName input').val() != "")&&($('.escalateFName input').val() != "")){
$('.Escalated_Approver_Name input').val($('.escalateLName input').val()+", "+$('.escalateFName input').val());
}
}
});