Both in water
Neither party has to describe a mood, because both register it on arrival. That removes an entire category of misunderstanding and is the reason this pairing feels immediately private and unusually close.
The cost of the doubling is the missing outside view. Air would introduce detachment and earth would introduce the practical facts, and neither is present. An atmosphere in the house therefore has nothing to check it, and a bad week can become a bad month without either person naming what happened.
Mutable and fixed
Pisces is mutable and Scorpio fixed, which is the load-bearing difference. Scorpio holds an emotional position at full intensity for as long as it takes. Pisces adapts to whatever position is in the room, including Scorpio's.
So the pairing is rarely a contest, and that is not entirely good news. Pisces accommodates rather than objects, Scorpio sets the emotional terms by default, and both can arrive at an arrangement neither actually chose. What Pisces does not say directly it expresses by withdrawing, which Scorpio detects and investigates.
Neptune, Pluto and the older rulers
Modern practice gives Pisces to Neptune and Scorpio to Pluto, both of them slow bodies concerned with what lies under the stated version of things. The traditional scheme is more informative: Jupiter for Pisces and Mars for Scorpio, one expansive and one pointed.
That older pairing explains the dynamic better. Mars pursues and Jupiter gives ground generously. Where Mars and Jupiter sit in the two charts, and how they contact each other, tells you more than the outer planets do.
Where it grinds
Suspicion against evasion. Scorpio assumes there is something under the surface and asks. Pisces, which does not lie so much as decline to be pinned down, gives a soft answer. That confirms Scorpio's assumption and the cycle tightens.
The second friction is practical neglect. Neither sign volunteers for administration, and both will let a serious matter of money or health drift while attending closely to the emotional weather instead.
What the suns leave out
Two water suns indicate a shared method of processing and nothing about compatibility of need. An enormous number of people have a water sun, and this pairing occurs across every possible outcome.
The Moon shows what each requires when unwell or frightened, Venus shows how affection is offered, and Mars shows what happens in an argument. Where either chart carries strong air or earth, the pairing usually gains the perspective it otherwise lacks. That is a fact about the charts, not the birthdays.
Questions people ask about this
Why do Pisces and Scorpio understand each other so quickly?
Both process by feeling, so neither has to explain a mood. The recognition is genuine and arrives early.
What is missing in an all-water pairing?
Detachment and practicality. Nothing in either temperament supplies an outside view or handles the administration.
Does Scorpio dominate this pairing?
Often by default rather than intent. Fixed water holds its position and mutable water accommodates it.
What should we check in the charts?
Mars and Jupiter under the traditional rulerships, then the Moon and Venus, and any air or earth placements in either chart.