Can A restaurant fire me for working at another restaurant.

Unfortunately . . . Yes.  Of course I am not an Illinois Attorney, but in New York and New Jersey, that is a perfectly valid reason for terminating an employee.  In NY, an employer doesn't even need to give you a reason.  But what an employer cannot do is use the excuse that you are working at another restaurant to fire you because he/she has really something against your race, religion, gender, nationality, disability, or sexual orientation (in some states).  Best bet is to call an Illinois Attorney.