I have a single task (of type Fixed Work) with a total Work estimated to 160 hours. When I assign two full-time resources (let's call them Alice and Bob) to it, Duration of the task is 80 hours, as expected. Neither Alice nor Bob have any other tasks assigned. However, when I change Units of Alice to 25%, Duration of the task changes to 320 hours.
This totally baffled me at first, but then I realized the reasoning (correct, I hope) behind it. Apparently, MS Project wants for each resource to spend exactly 80 hours on the task. Since Alice can only spend 2 hours (25%) each day on the task, it takes her 40 days to ramp up 80 hours and that is the reason Duration of the task is 320 hours (= 40 days).
Is there a way for Bob to be able to spend more than 80 hours on the task, so that he can compensate for Alice? In other words, I would like the resulting Duration to be 144 hours (= 16 days, during which Bob works for 8 hours each day and Alice 2 hours).