I have 2 identical source PC which connect to VP1920 and display on 2 displays.

When I switch source for display A, display B also blacksreens while display A switches source.

Is it normal?

About the switching, it's behavior is normal because of the design.  But there is a situation that wouldn't have the flashing. 

For example, if display A displays source 2 and display B displays source 3, there is no flashing if you switch source 1 to display A or B(different source, not source 2 and 3).

But if you switch source 2 or 3 on display A or B, it will flash.