An argument that Merge is binary but its binarity refers to syntactic positions rather than objects.
In this book, Barbara Citko and Martina Gračanin-Yuksek examine the constraints on Merge—the basic structure-building operation in minimalist syntax—from a multidominant perspective. They maintain that Merge is binary, but argue that the binarity of Merge refers to syntactic positions Merge relates: what has typically been formulated as a constraint that prevents Merge from combining more than two syntactic objects is a constraint on Merge's relating more than two syntactic positions.
Citko and Gračanin-Yuksek investigate the interactions between the two types of Merge that can generate multidominant structures: Parallel Merge and Internal Merge. Taking Right Node Raiding (RNR) as a representative example of Parallel Merge and Across-the-Board (ATB) extraction to be representative of Parallel Merge + Internal Merge, they show that ATB is subject to a parallelism constraint that RNR is not subject to. They show that this difference follows from Binarity Constraint on Merge (BiCoM), the requirement that prevents Merge from relating more than two syntactic positions within a single derivation, which is obeyed in RNR, but not in ATB extraction. They further show that BiCoM is also operative in languages with more flexible word order, such as Croatian and Polish, and that structural syncretism alleviates BiCoM violations in these languages as well.