The term "co-joined" typically refers to two or more things that are joined or connected together. The best antonyms for co-joined are words such as separate, disconnect and disjoin. If two things are separate, they are not joined together in any way. Similarly, when we disconnect or disjoin two things, we remove the connection between them, so they are no longer joined. Other possible antonyms for co-joined include unattached, divided, and split. All these words suggest a lack of togetherness or a disconnection between two or more things that would otherwise have been joined.