It's definitely not more productive per hour than 40. And if you're moonlighting another 30 hrs that'll definitely decrease your productivity at work.
And I don't mean productivity per hour. Lol. No, I mean absolute.
An employee working like a dog will get less work done than one just working normally, probably. Because most of the work is negative, so it doesn't add to the work done pile, it chips away at it.
Eventually, I would think, you reach a point where an employee is less productive than no employee at all. Seems impossible to be working 100 hours a week and be getting less than nothing done, but if you're actively making the product worse or creating debt, that's how I would classify that.