Achieving inclusive mathematics education remains a distinctive challenge due to the discipline’s hierarchical structure, high degree of abstraction, and dense symbolism. These disciplinary features create unique cognitive barriers for students with special educational needs, requiring a precise intersection of content expertise and pedagogical scaffolding. While collaboration between mathematics and special education (SPED) teachers is vital to dismantling these barriers, the success of such teams is mediated by the teachers’ mental models—the internal assumptions that dictate their collaborative engagement. Utilizing a multi-case study design, this study examined teacher mental models through cross-disciplinary Lesson Study (LS) focused specifically on making abstract mathematical concepts accessible. Based on data collected before, during, and after four LS cycles, findings reveal that participants’ initial mental models of collaboration were limited to rudimentary idea-sharing. However, the LS experience facilitated a shift toward reciprocal learning, where participating teachers engaged in a mutual exchange of disciplinary expertise to co-create lesson resources and strategies. This shift is framed as a theoretical insight into cognitive syncretism: the restructuring of compartmentalized professional identities into a shared, integrated schema. Specifically, SPED teachers acquired the technical mathematical depth necessary to navigate cumulative curriculum demands, while mathematics teachers adopted differentiated strategies to decode complex mathematical symbols for diverse learners. Ultimately, this research positions mental models as primary mediators of educational reform, suggesting that sustainable inclusion in mathematics requires a fundamental cognitive reorganization of how teachers perceive the intersection of mathematical rigor and instructional accessibility.