A co-primary is a person who is one of two or more primary partners in a polyamorous relationship. For example, Bob has two primaries: Jill and Annie. Jill and Annie are co-primaries in this relationship.
More About Co-Primary
In most polyamory relationships, there’s usually one primary partner and several secondary and/or tertiary partners. In certain cases, a person can have more than one primary; a triad (three people dating each other) is an obvious example.
A primary is a partner whom you consider a top relationship priority. It can also refer to the partner with whom you have the most financial/emotional involvement (as in a marriage). In any case, the health of a primary relationship usually comes first, especially when it comes in conflict with secondary or tertiary relationships.