This article reports on the design of interdisciplinary units in five International Baccalaureate schools in Norway and Denmark, each with fewer than 100 students in their Middle Years Programme. The mixed methods study describes subject combinations and time frames of 37 enacted units and 111 hypothetical interdisciplinary units, and the qualitative perspectives of nine teachers, representative of the curriculum frameworks' eight subject groups, on the contribution of these school-based units to student learning. In the special context of schools with logistically simple staffing structures, interdisciplinary units were consistently conceived and enacted as substantial units, typically implemented over 8 weeks. Subjects were usually paired using a common Key Concept and differentiated by Related Concepts. Although framing and classification were considerations in interdisciplinary planning, the quantitative data found nearly every pairing of the subject groups was feasible. Interdisciplinary units were consistently valued by teachers for helping students connect disciplinary knowledge and developing metacognitive skills that supported their future learning, and for enriching teachers' own pedagogical practice. As a pragmatic, sustainable approach that resolves the tensions posed by assessment of disciplinary and interdisciplinary perspectives, the strategies reported in this study suggest a 'middle way' for countering the disciplinary fragmentation associated with junior secondary education.