The proliferation of JD/PhDs over the past generation has resulted in many junior faculty candidates facing the question: should I seek a "joint" appointment between the Law School and the cognate PhD discipline?
"Joint" appointments come in various forms, of which the two main ones are: (1) tenure-track status in two units, with two separate tenure reviews, and two separate tenure decisions; and (2) a "courtesy" or "secondary" appointment in the cognate department, with the tenure home residing in the Law School. The former is, I suppose, a fully "joint" appointment, but it is also to be avoided (perhaps even after tenure, since it is likely to increase your administrative burdens [committee work, faculty meetings etc.]). Although it's still easier, alas, to get tenure in a law school than in most academic departments, the bottom line is having two different tenure masters is a bad position to be in. (There are cases of faculty who didn't get tenure in the non-law department, but did get it in the law school, and those situations are unhappy ones all around.) On the other hand, (2) can have benefits for the faculty member (perhaps teaching in the cognate department, involvement with PhD students and the like) without any of the costs.
But a JD/PhD on the rookie law market should be careful about raising the question of courtesy appointments. Law schools understand full well that they offer better terms of employment (in teaching load, salary, and research support) than almost every academic department in the humanites and social sciences, and so a key question for them in hiring JD/PhDs is: why do you want to be in the Law School rather than in the cognate field? The answer had better turn on intellectual and pedagogical considerations. After a JD/PhD has an offer, you can raise the question about courtesy appointments (assuming they exist, not all schools have them), but if you're hired by a Law School, do understand your primary obligations reside there.