Two Virgos, two Mercuries
Mercury rules Virgo, and Mercury stays within twenty-eight degrees of the sun, so a Virgo sun carries Mercury in Leo, Virgo or Libra. A Virgo with Mercury in Leo argues in broad declarations and takes correction as a slight. A Virgo with Mercury in Virgo argues in specifics and treats correction as courtesy. They are recognisably different people, and every compatibility shortlist assigns them the same four partners.
That is the first and largest objection to the method. It cannot see inside the sign it is sorting.
Where the four names come from
Capricorn and Taurus are the other earth signs, each standing 120 degrees from Virgo. Scorpio and Cancer are water, 60 degrees off in either direction. Antiquity ranked those two separations as favourable, and every list of easy signs for Virgo since is just a reading of which signs occupy them.
Note the direction of the reasoning. It runs from the angle to the sign and stops. Nothing about the reputation of Virgo, or of Capricorn, enters the calculation at any point.
Mutable earth, which is a stranger combination than it sounds
Virgo sits at the close of a season, so it is adaptable, but its element is the one that resists adaptation. The result the tradition describes is a sign that adjusts endlessly within a fixed set of standards: methods change, criteria do not. Against fixed Taurus that reads as one party revising and the other holding; against cardinal Capricorn it reads as division of labour by rank.
These are descriptions of how the categories sit together. Whether a given pair of people find that arrangement comfortable is not something the categories can say.
What actually carries information
A sun sign is a single placement in a chart containing many. A comparison with substance reads both Moons for private requirement, both Venus placements for how affection is offered and recognised, both Mars placements for the shape a disagreement takes, and both ascendants for what each person meets first. Then it measures the aspects running between the two charts rather than inside either, which is synastry, and the synastry calculator does that from two birth times.
Even so, no result identifies a soulmate. The word has no technical sense in astrology; there is no aspect for it, and the classical writers never reach for the notion.
Read the combination, not the ranking
If a particular sign is on your mind, that pairing has its own page and it sets out what the combination handles well and what it wears through. Virgo and Sagittarius, which no list recommends, is the more useful read, because a page about friction is obliged to be specific.
With two birth times available the charts can be compared directly. That answers a smaller question than the one most people came with, and answers it properly.
Questions people ask about this
Which signs does tradition name for Virgo?
Capricorn and Taurus in earth, Scorpio and Cancer in water. Both groups follow from the angle rather than from evidence about people.
Is there an astrological sign of a soulmate?
No. Nothing in the chart carries that meaning, and the idea was imported from outside the subject.
Why does Mercury matter for Virgo?
It rules the sign, and for a Virgo sun it falls in Leo, Virgo or Libra, which changes how the person reasons and how they take correction.
What does a serious comparison read?
Moon, Venus, Mars and ascendant in both charts, plus the aspects measured between the two.