They do not knowingly allow two accounts for the same person except as exceptions. When they discover you have two accounts they merge them. They are strict because of possible fraud: claiming the same flights on two accounts. When my two accounts were merged they claimed to do a flight comparison to ensure I had not claimed double credit. I suppose many a high mileage flier is doing this to get extra SWUs, but I don't know what penalty they would impose if discovered and you had obtained additional SWUs this way.