For example, if he spends 20 hours a week on his advisor's project, and 20 hours a week on a project with another professor (that the advisor is not involved in). Do most advisors find this acceptable, or do they see this as taking up work time that belongs to them?

And what if those 20 hours a week are spent on non-research activities, such as taking classes unrelated to the topic of the PhD, or a part-time job outside the university?