Co-join, meaning to join or connect together, can have a variety of antonyms depending on the context. If co-joining refers to merging two entities, the antonym might be to disconnect or split apart. In a more general sense, antonyms could include separate, divide, or disassociate. These words suggest a breaking apart rather than a combining of elements. Alternatively, antonyms for co-join might be found by considering the opposite of different forms of connection, such as incoherent, inarticulate, or disorganized. Ultimately, the choice of antonyms for co-join will depend on the specific context and underlying meanings of the text.