Is it legal that we be required to supplement a co-workers income and why?

Generally, an employer may pay you at any rate they chose, so long as you are paid minimum wage or more. Also, employer may, at their whim, change the pay structure, so long as you are paid minimum wage or more. I suggest you consult Wage and Hour Division (WHD) - U.S. Department of Labor http://www.wagehour.dol.gov/ They have investigators that handle such questions. Your question does not identify your state. This response is based on Georgia law. If you are in a state other than GA, resubmit your question, and identify your state. The laws are different in other states, but the Wage and Hour laws apply in every state. Your remedy may be to find another job.