In Mississippi is it legal to have one employee working for two companies owned by the same person, but only be compensated by one company.

Is the employee being compensated for all the hours he worked?

yes and no. It's doing two different jobs at the same time. So basically I work for the main company full-time (8 - 5) and probably 3 or 4 hours out of the day I am doing work for both companies simultaneously.

As long as you are being paid from 8-5 (assuming you are a nonexempt) employee, it doesn't make any difference that one of the companies is paying your entire pay, even though part of your day is spent working for the other company.

And if you are an exempt employee, it still doesn't make any difference, as long as you get your guaranteed weekly salary from one of them.