The reason employers would not approve isn't because the employee's attention is split, but because the employee would not be meeting the conditions of employment!
40 hours is the commonly accepted definition of "full time" in the US, and this is often written into employment contracts as well. Additionally, employers generally have expected working hours where the employee is available for meetings, mentoring, pair programming, etc.
So, how can you fulfill the expectations of multiple "full time" jobs while only putting in the time for one?