20 CFR § 345.403 - Multiple base year employers.
(1) If the employer at the time of the claim is the same as the last employer in the base year, benefits will be charged in reverse chronological order, but the amount charged to each base year employer shall not exceed the amount of compensation paid by such employer to the employee in the base year;
(2) In all other cases, benefits will be charged in the same ratio as the compensation paid to such employee by the employer bears to the total of such compensation paid to such employee by all such employers in the base year; benefit charging in accordance with this method shall apply whether the base year employment was with successive employers or with concurrent employers.
(b)Excess benefit payments. If, in applying the rule in paragraph (a)(1) of this section, there remain benefit payments, in whole or in part, that cannot be charged to any base year employer, the amount of benefits paid in excess of those chargeable under paragraph (a)(1) shall be charged to the system unallocated charge balance.
(c)Board records as basis for charging multiple base year employers. Where an employee has more than one base year employer, the Board will use records compiled on the basis of employer reports filed under § 345.110 of this part for the purpose of determining whether the employer at the time of the claim for benefits is the last employer in the base year and for other purposes related to benefit charging under this subpart. If, in a particular case, such records do not contain all the data necessary to determine the charge, the Board will request the necessary data from the base year employers who may be liable for the charge.