Legal Question in Employment Law in California

You should not be working for free. If you provide labor for any entity, you be getting paid for it.

It actually depends on whether or not you are an exempt employee or hourly, and whether you are paid byt the first company while doing work for the second. Your employer is perfectly free to have you do work for another company while on the first company's payroll. Entire businesses are based on that model, such as business consulting firms. If you are non-exempt, as long as you are paid for the hours you work by one or the other, and overtime if you qualify, you aren't entitled to extra pay from the other company. If you are exempt, as long as you are paid your salary you are not entitled to additional compensation.