I´ve successfully managed to play up to 8 videos in sync using a single video window with multiple streams using the directshowlib for c#. The problem is the video window pl
I assume that you're using the VMR with multiple input pins. The VMR is going to render to a single surface, which needs to be on a single display. You should be able to render your streams to multiple VMRs, with each VMR placed on a separate display within your maximised window.
It sounds as though you have all the streams in a single graph. You can separate them into different graphs, with each graph having one source and one renderer. Starting the graphs in sync means using IMediaFilter::Run instead of IMediaControl::Run: