The paper introduces a type of joint utterance construction in Japanese, in which two independent sentential-level units are amalgamated, which has hitherto received little attention in the literature. Unlike traditional joint utterance construction where one speaker maintains authority over the syntactic structure of the forthcoming continuation and the other accedes to this, thereby constituting a single TCU (turn constructional unit), our examples demonstrate that both speakers can have authority over the syntactic design of joint utterances. We call such collaborative utterances ‘co-authored joint utterances’ in this paper. The uniqueness of co-authored joint utterances lies in their syntactic architecture. While syntactic and semantic continuity are successfully achieved in constructing co-authored joint utterances, they represent a co-joined structure in which two sentential-level units are involved with their shared part constituting a point of amalgamation, and because of this, the structure of a co-authored joint utterance can no longer be parsed with extant grammar. In analysing co-authored joint utterances, we examine how they can be treated in relation to the distinction between TCU (Turn Constructional Unit) continuation and new TCUs. Due to the particularities of the syntactic architecture of co-authored joint utterances, their existence raises questions about the way in which this distinction is currently operationalised, because despite being syntactically an incremental continuation, and so seemingly a TCU continuation, the co-authored joint utterance implements an action beyond what was initially instantiated by the antecedent of that joint utterance, and so arguably constitutes a new TCU.