You may be thinking of contract work, though contract work could have regular working hours associated with it, too. When an official employee is hired to work at a set schedule, they are expected to work for that employer and only that employer while on the clock. If you are getting paid by two employers during the same hours while on the clock, that is known as double-dipping. It is a crime in the U.S. It is theft.

I think you are trying to rationalize this as being "productive". It is not a matter of being productive. He is being paid for time with the expectation of production. If he is a contractor getting paid to produce x-number of widgets, then when he meets his quota, he has satisfied his contract. If he is getting paid for 8 hours a day and he is working someone else during those hours, he is not meeting his requirements, no matter how many widgets he produces.