Generally, employees are not obligated to disclose to their employers that they have a second job. However, in recent times, employers have included a clause in employment contracts to restrict employees from having another job. In such a case, an employee will be in violation of their employment contract if they take up another job while still currently employed.
Of course, if you’re discreet about it, it’s unlikely the employer will find out. However, this doesn’t change the fact that it’s a violation of your employment contract.