If you're working for another company, typically secretly or in night, in addition to your real job, it's known as moonlighting.
And legally, I don't think it's illegal
Though it's bad for a company, especially if you're working for the rival secretly
And of course, you'll lose your job as soon as you'll get caught
In fact, I just received a news that Wipro fired 300 employees found moonlighting for competitors
And, even for the employees, it's bad
As in short term, you'll see its benefits, such as more income, but tbh, if you don't have control over your money, additional money can't do anything. You'll eventually find yourself in rat race, and by working too much, one's mental and physical health gets affected.