D.C. Mun. Regs. tit. 9, r. 9-140
If a payment of wages is made to an employee by an employer through an agent, fiduciary (or other person who also has the control, receipt, custody, or disposal of, or pays the wages payable by another employer to the employee), the amount of the tax required to be withheld on each wage payment shall be determined upon the aggregate amount of that wage payment or payments in the same manner as if the aggregate amount had been paid by one employer.
The determination made in accordance with § 140.1 shall be made without regard to whether the wages are paid separately on behalf of each employer or paid in a lump sum on behalf of all employers.
Each employer shall be liable for the return and payment of a pro rata portion of the tax. In other words, each employer's portion shall be based on the ratio of the amount paid as wages by that particular employer to the aggregate of all wages.
D.C. Mun. Regs. tit. 9, r. 9-140