"so the agency sent him a W-2 that is combined."

"And the agency won't correct or split W-2s since it is in their right to combine since they were the payer."

"That's correct -- same EIN, no excess FICA."

From: https://www.irs.gov/newsroom/employers-use-of-payroll-service-providers

Reporting agent:

"Prepare Form W-3, Transmittal of Wage and Tax Statements, and file and furnish Forms W-2, Wage and Tax Statement, for the employees of the employer using the employer's EIN."

Section 3504 agent:

"An agent appointed under Form 2678 files aggregate returns (e-file or paper) using the agent's EIN."

Certified professional employer organization:

"A CPEO files aggregate returns using the CPEO's EIN."

Someone is doing something wrong. Either they are supposed to also work on their one, combined, gross wage for FICA and one W2; or, they are doing it for the individual employers, resulting in two separate W2 forms being issued. It seems they are coloring outside the lines of whichever functions apply to their actual, contractual, designation.

Their excuse doesn't make sense.