Make sure supervisors know of the situation, especially if the employee in question feels overloaded. All things being equal, try to spend 1 day on one project and 1 day on the other, alternating. Do not work more than your average number of hours per week. You are unlikely to be rewarded for working free overtime.
Sometimes you can use one project as relief for the other, when you're stuck.
@progo @CSB I have been in this situation before. At one point I was a "fully dedicated employee" for 4 separate customers...each thought they were paying for my full time services for themselves only. Obviously I only saw a fraction of that income personally.
If your colleague is not able to have open communication with the PM for each project, then he has to go above their heads and make sure upper management knows that both projects will suffer.
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