As soon as you use several projectors for use as a large, seamless desktop, having them shown as separate displays in Windows creates several issues. Apart from missing synchronization, the behaviour of any application is not like you would expect - they are not automatically expanded across the entire desktop.
Therefore combining all projectors to a single locical screen for the "eyes" of the operating system is a must.
The feature for this is provided by NVIDIA GeForce graphics cards as "Surround". It is provided by the NVIDIA Control panel.
This online-guide shows how to create such a Surround setup:desktop spanning, nvidia, surround