Does the concept of nepotism describe the hiring of both members of an academic couple by one university? Yes, I suppose so, even though neither member of the couple is the one making the hiring decision. It's not the same kind of nepotism as when John F. Kennedy appointed his brother Bobby as Attorney General. Nevertheless, it does involve at least one person in the couple's being hired because of their relationship to another.
In my field, there are rare examples of parent-offspring pairs in related fields of research, and I suppose that the famous name is a plus in some ways for the offspring, but I've seen it backfire for father-daughter pairs (the usual sexist rumors that he is really the brains behind all her research). Most often, the situation involves a couple, whether a married couple or long-term partners.
Another variable is whether both members of the couple are being considered simultaneously for hiring, or whether one has already been hired and the other is considered later (i.e., in the case of one member of the couple's not yet being done with his/her Ph.D. at the time the other was hired). The latter situation can be hard on the non-employed member of the couple, and these days still seems to require that a competing offer for both be obtained in order for both to get academic positions.
Nepotism is unavoidable if women are going to be hired in significant numbers at academic institutions. It's important to note that nepotism in this sense involves both the case of a woman offered a job by an institution that is primarily interested in hiring the other member of the couple (typically a husband) and the case of an institution wanting to hire a woman and also offering a position to her partner/spouse. In some cases, the university administration provides funds for the extra position, either permanently or as a 'bridge' to the next retirement in a department. Difficulties arise when departments have to fund the position, the positions are in different academic units of a university (and the higher level administration doesn't help facilitate the hires), and if both members of the couple require large amounts of start-up funding.
In most cases that I am aware of, the 'extra' person is not taking away a position from a 'more deserving' person out there in the applicant pool. It is important not to get into the mode of thinking that unqualified people (substitute name of underrepresented group) are taking jobs away from qualified people (substitute name of dominant group).
However, the Kennedy example aside, the concept of nepotism typically implies that a person being appointed is selected despite their lack of qualifications or abilities, and that's a source of yet more trouble for academic couples.
Most departments and universities hire faculty with the expectation that they will succeed -- faculty will get tenure and do well with whatever the research-teaching-service expectations are for the institution. So what to do if there is an imbalance between the qualifications of members of an academic couple? That is, what if one is qualified to be hired at a major research university and the other isn't?
I don't have The Answer. I don't think it's in anyone's interest to hire an unqualified person in a tenure-track position at a research university, but I think that the definition of 'qualified' needs very careful consideration in these cases. It needs extremely careful consideration, so as to be sure that the usual biases about what makes a successful scientist are not clouding the evaluation of a promising female scientist.
Another pitfall is hiring one member of the couple in a tenured or tenure-track position and the other in a low-level, low-respect position. That doesn't do anyone any good either -- not the couple, and not the department/university.
I stand by my statement that nepotism is required for women science/engineering faculty to be hired in significant numbers at academic institutions, particularly at universities and colleges that are rather isolated, leaving few to no other opportunities for alternative academic employment by one member of the couple. There's no reason, however, why nepotism has to be forced into the existing academic culture without the possibility of changing the culture. Why can't the culture itself change? Why does it have to be an all-or-nothing situation in which you get on the tenure track and either succeed or not in a certain amount of time? Why do non-tenure track positions have to be associated with so much less respect and so much less chance of career advancement? Is there room for other types of positions with different expectations in terms of the balance of teaching-research-service? The academic culture needs to change, and it needs to change at all levels -- administrators need to be creative, and faculty need to be open-minded.