You could copy your email server behavior so it sends to the two recipients separately and only use logging for one or the other.
You could combine the fields like:
$Email->onSend->Query->bindColumn("FullName", "s", "".((isset($_POST["First_Name"]))?$_POST["First_Name"]:"") ." " . ((isset($_POST["Last_Name"]))?$_POST["Last_Name"]:""));
$Email->onSend->Query->bindColumn("LastName", "s", "".);