This study examines students' visual behaviors when they tackle two types of synthesis problems, sequential and simultaneous problems. Sequential synthesis tasks can be solved by applying pertinent concepts consecutively, whereas simultaneous synthesis tasks require concurrent application of multiple concepts. Twenty-two students from an introductory calculus-based physics course participated in the study. We used an eye-tracker to record the students' eye movements when they silently reflected on how to solve the problems and subsequently when they talked aloud their problem-solving strategies. We found that the students made more gaze transitions between text and diagram for the simultaneous problems than for the sequential ones. However, they spent more time looking at the diagram and making within-diagram eye transitions in the sequential tasks than in the simultaneous tasks. Further, most students invoked two concepts to solve the sequential tasks but only one for the simultaneous tasks. These findings indicate that the students made less effort to link text and diagram in solving sequential problems but frequently attempted to integrate information within each diagram. The pattern for the simultaneous problems appeared to be reversed. Our results suggest that different types of synthesis (i.e., sequential and simultaneous) may differentially influence the ways students handle tasks. As suggested by the eye-tracker data and confirmed by the participants' verbal explanations, students tend to divide the situation in sequential problems into subtasks but treat simultaneous problems as a single-step task.