Two strange bedfellows
There are a bunch of things to consider concerning Neptune and Pluto relative to a relationship.
First, these are generational or collective planets, not personal planets. By this I mean that personal or inner planets (Sun outward to Saturn) make frequent and short-lived aspects to each other -- measured in hours or days as a rule. Thus they bring dynamic change into our charts and lives. The outer planets (Uranus, Neptune, Pluto plus any Trans-Pluto or Uranian Hypotheticals) slowly move into and out of aspectual contact with each other -- measured in weeks, months and even years -- and manifest more in world events that develop and emerge over time.
Second, when a person's inner planets aspect an outer planet of theirs or of another then that planet's social or broader influences are integrated and expressed through the person's inner planet. For example, Neptune conjunct the Sun might indicate seeing a social cause as a personal interest or activity, or drive one to seek to clarify and objectify a dream or collective ideal -- I'll try to explain and justify the use of Marijuana for medical purposes to the US Congress so they'll change the federal drug laws.
Third, when a relationship is built upon or centered around aspectual links between Neptune and Saturn then there may be a problem. Alone, these two are not the right basis for most relationships. Together, these two could suggest a relationship in which one's ideals and motivations were shaped by the intensity and power drives of the other. Understanding the deeper under-pinnings of a social concept that one of the partners has would cause the other to become subserviant or blind to some aspects of life other than the primary cause that fueled the relationship. What would be truth, mystical belief, fantasy, leadership?
It is sometimes a life's challenge to integrate either of these two into a full and rewarding personality. It may be too big a challenge to cross-integrate these two within a couple's duality and individuality without a large sacrifice or common, persistent goal to pursue. Dave.