This study explores boundary-crossing interaction between two grade 5/6 science classrooms that operated as knowledge building communities. The two classrooms studied human body systems with the support of the Knowledge Forum over a ten-week period. The knowledge building practice integrated student-driven inquiry and discourse within each community and cross-community interaction mediated through "super notes" posted in a cross-community meta-space. Students co-authored super notes as epistemic boundary objects, each of which synthesized knowledge progress in an emergent line of inquiry for cross-community sharing. Qualitative analyses of classroom videos, online discourse, and interviews provide a rich description of how the students conceived, generated, and interacted with the super notes for knowledge building. The processes to transcend student ideas toward the higher social levels for sharing through boundary-crossing further served as a larger context for idea development toward higher epistemic levels. Incorporating cross-community interaction is important for scaling CSCL-based classroom practices in a way that fosters high-level epistemic engagement.