Sollux is the technical parent: He knows how to handle the health problems, handles the diapers, builds the toys and furniture, makes the bottles, knows how to put a baby to sleep if they cannot do so themselves
Karkat is the physical parent: He plays with them, rocks them to sleep, sings them lullabies, feeds them, supportive, teaches them new things they will possibly need to know, extensive cuddler.
Both are great parents: They support their children, ensure they are set for the future, worry, care, punish when needed, love and everything in between.
But to be good parents they need each other, because their parenting styles balance each other out.
Of course for shopping needs they request the assistance of Kanaya. And Jane makes the cakes. Sollux can cook, but like hell he could make a cake.