I'm not a solicitor but the primary carer is the parent who cares for the children for the majority of the time. Evidence of that are the established patterns of work/childcare such as who takes the children to and from school, nursery, doctors, dentist, shopping and who does the day to day care such as washing and cooking the children's clothes.
It seems in your case both parents have historically cared for the children and in those circumstances shared residence might be appropriate. Shared residence is when a child "lives" with both parents but it can be in different proportions from 50:50.