(from the perspective of an untenured Assistant Professor) In January, a student approached you to be his thesis advisor, and you agreed. During the subsequent semester, you found that he was taking more of your time then you were really able to give. He was not at a point where he could effectively contribute to any of the calculations that you had in hand. He would frequently give you his own calculations for comment and, because of other time constraints, you usually were not able to read them in a timely way. While you would have preferred a student with more independence, you thought that the situation might improve over time.
In May, a senior colleague in your group left and his advanced graduate student opted to stay at your university rather than accompany your colleague. You were pressured by your department to take this student. As the summer progressed, it became clear that you could not effectively deal with both students and do all of the research that was needed in order to present a good tenure profile. Your options seem to be to cut the first student loose or to keep both students.
What should you do?
What points would you consider in your decision-making?