In some scenarios, we need to combine multiple video streams and pictures into a single video. In this paper, we develop an application to synthesize two channel video stream and two channel pictures. Our application can be used for synthetic display of multiple network video streams and offline video synthetic storage. As the single thread decoding of multiple video streams takes a lot of time, we propose the use of a multithreading producer-consumer pattern. It makes full use of the advantages of multi-core CPU and improves the programming efficiency. In our experiments, we compare video synthesis through single thread and multithreading. Multithreading is much faster than single threading.