I know several people who spent months working for two companies: one full time, the other part time. The most productive few would reach two full time positions and actually keep delivering for over a year.
The reason this happens at all is that sufficiently large organisations expect performance to be in a specific range - if it's too low you'll be fired, but going the extra mile will not yield benefits, as your compensation is decided by the assigned budget and promotions are rare.
Case in point: a few years ago my former co-worker was given "overtime" which was actually a hidden raise, as management really wanted to keep him, but couldn't officially increase his compensation. The organisation for which we worked eventually cracked down on such practices, so he left to work at a place which would compensate him this much and more without resorting to such tricks.