The implementation of bioassays in pin-constrained digital microfluidic biochips may involve pin-actuation conflicts if the concurrently-implemented fluidic operations are not carefully synchronized. We propose a two-phase optimization method to identify and synchronize the fluidic operations that can be executed in parallel. The goal is to implement these fluidic operations without pin-actuation conflict, and minimize the duration of implementing the outcome sequence after synchronization. We also extend the synchronization method with the addition of a small number of control pins, in order to further minimize the completion time while avoiding pin-actuation conflicts. The effectiveness of the proposed synchronization method is demonstrated for a representative 3-plex assay performed on a commercial pin-constrained biochip and for multiplexed in-vitro diagnostics performed on an experimental pin-constrained biochip.