An employee usually performs services for only one person or firm at a time. However, a person may work for a number of people or firms and still be an employee of one or all of them because he or she works under the control of each firm. For example, a bookkeeper may perform services for four firms, 2 hours every workday on each firm's premises, and be controlled and directed by all of them.

A person who works for a number of people or firms at the same time is ordinarily not an employee. Work for a number of people or firms at the same time usually indicate an independent contractor status because the worker is free from control by all of the people or firms.

Social Security Handbook, section 820 : Do you work for more than one person or firm at the same time?