If fatigue is a simple threshold (i.e. you "become fatigued" after a certain number of units of work done), then the obvious solution is to work the employees at 100% utilization
for half as long (at which point they should be roughly as fatigued as the employees worked half as hard for twice as long), and then swap them out for a second shift. (I.e., instead of having 10 employees with 40hr weeks, you now have 20 employees with 20hr weeks.)
This is the approach I believe is also advised for improving productivity/decreasing errors in the medical field (where doctors have a hard time not pushing themselves to 100% all the time, because of their personalities): hire more doctors per hospital, such that each doctor can be cut down to a shorter shift (and thus maybe lower salary, too.)
If, on the other hand, fatigue is about the lack of micro-rests between individual units of work, then the appropriate solution is more subtle:
1. replace the low-quality rest of doing "slack" work, with high-quality rest of doing nothing-at-all (or even "actively" resting, the equivalent of an athlete doing cool-down stretches between sets of an exercise), so that employees can cool down more quickly and/or return closer to peak productivity from each rest;
2. have slightly more employees (hopefully fewer than double), working off branching lines, such a way that units of work are directed to go to whichever employee is "fresh" rather than "stalled." Like processors with pipelined executions and multiple APUs per core, directing instructions to the APU that isn't currently blocked.