You can watch Unreal Engine 4, Multi Window c++, Blueprints and UserWidget - YouTube, Basically you need add actor MultiWindow/Content/Blueprints/BP_MV_SpectatorWindowActor to scene and after it It will automatically create new window with same texture from frame buffer.
Also you can setup different resolution for window and for UserWidget Texture in Blueprints SpectatorWindow/Plugins/MultiWindow/Content/Blueprints at master · Batname/SpectatorWindow · GitHub
That plugin In development, But you can take Ideas from C++ and Blueprints for your requirements