Conn. Agencies Regs. § 12-705(a)-7
If payment of wages is made by an employer through an agent, fiduciary, or other person who also pays, or has the control, receipt, custody, or disposal of, the wages payable by another employer to the same employee, the amount of the tax required to be withheld on each wage payment to be made through such agent, fiduciary, or other person shall, whether the wages are paid separately on behalf of each employer or in a lump sum on behalf of all such employers, be determined upon the aggregate amount of such wage payment or payments in the same manner as if the aggregate amount had been paid by one employer. In such a case, each employer is liable for the return and payment of a pro rata portion of the tax so determined, such portion to be determined in the ratio which the amount contributed by the particular employer bears to the aggregate of such wages.
Conn. Agencies Regs. § 12-705(a)-7