I don't know how other states work, but when you report Alabama wages to the labor department, you do it on their web site and you just put in the amount the employee earned for that quarter. They already have the wages reported for earlier quarters for that employee from all places worked and they figure the tax for you and deduct the amount from the employer's bank account. They no longer allow the employer to send in a check no matter what the amount is. Of course FUTA could be a problem if two payroll agencies prepare a 940 for the same employer.
Edit: Also, in Alabama if an employee works for two employers, the $8,000.00 limit starts over. I assume that is true in all states.