Earth on both sides
Earth deals in what can be measured, and doubled it produces a household that functions. Money is handled, obligations are met, promises are kept because a promise is treated as a debt. Very few pairings are this reliable.
What is absent is lightness. Neither party initiates play, neither treats leisure as anything other than deferred work, and the relationship can become an efficiently run administration with two competent people in it and no warmth passing between them. It is a slow failure and it is easy to miss while it is happening.
Cardinal doubled
Capricorn is cardinal, which means it starts, and specifically it starts long structures. Two of them will build ambitiously. The pairing usually acquires property, careers and responsibilities faster than its peers.
The difficulty of doubled cardinal is authority. Both parties assume they are the one with the plan, and Capricorn does not negotiate the plan, it presents the completed version. Two completed versions of the same plan is the recurring domestic argument, conducted politely and without resolution.
Saturn against Saturn
Saturn rules both charts, which means both people are governed by the body of limit, duty and delay. That is a heavy weight to have on both sides with nothing to offset it.
Saturn also describes what each of them is hard on, and its condition in each chart matters greatly. Two well-placed Saturns produce endurance and self-respect. Two afflicted ones produce two people who withhold approval, from themselves and then from each other, and call it high standards.
Where it grinds
Emotional expression, and the contempt for needing it. Capricorn treats vulnerability as a lapse of discipline, so neither asks for comfort and neither offers it unprompted. Difficulty is carried privately by both.
The second friction is status. Capricorn measures itself against a standard, and with a partner of the same sign the comparison becomes internal to the relationship. Whose career, whose family, whose judgement carries more weight is rarely stated and constantly present.
Same sign is not the same chart
Two Capricorn suns share an approach to work and time. They do not share a nervous system. One with an Aquarius Moon and Venus in Sagittarius runs quite differently from one with a Scorpio Moon and Venus in Capricorn, and daily life is set by those placements rather than by the sun.
A shared sun sign is an observation about a huge population and a poor description of two people in it. The Moon, Venus and Mars comparison is where the useful information sits.
Questions people ask about this
Is a Capricorn and Capricorn pairing too serious?
It can be. Neither partner initiates rest or play, so the relationship trends towards administration unless something in the charts counters it.
What does doubled Saturn do?
It doubles the discipline and the tendency to withhold approval. The condition of Saturn in each chart decides which effect dominates.
Do two Capricorns compete with each other?
Usually quietly. Both measure themselves against a standard, and with the same sign opposite them the measuring turns inward.
What is worth checking beyond the sun?
The Moon in each chart above all, since neither sun sign asks for comfort. Venus and Mars follow.