Co-teaching, a partnership between professional peers with different expertise to jointly deliver instruction and divide teaching responsibility, is recognized as an effective teaching strategy that has been widely implemented. The increased use of information and communication technologies in educational practices may expand the opportunities for potentially beneficial teacher collaboration across schools. How the online teacher and the on-site teacher co-teach in blended synchronous teaching and learning, as well as its effectiveness on student engagement, remains unclear. This paper presents the results from a sequential research design from the teaching presence perspective to shed light on the characteristics of co-teaching presence and its effect on student engagement in the blended synchronous classroom. In study one, qualitative data collected through ethnographic observation and interviews exhibited how the co-teaching presence was created and its elements: instructional design and organization, facilitating discourse, direct instruction, assessment, supplementary instruction, organization and management, and affective support. Study two was a quantitative study that applied a self-report questionnaire to 268 students to further verify the effects of elements of co-teaching presence on student engagement. Based on hierarchical regression analyses, the results provided evidence that student engagement benefits from collaborative teaching; the on-site teacher's affective support had the greatest influence on sustaining student engagement. Additional findings, implications, limitations, and research directions are discussed.