Hello,
as far as I know there can only be a single GameInstance at once (therefore called a “Singleton”) that can be accessed by any Object/Actor with a UWorld Context. Therefore I do not think you can get multiple GameInstances (at least not without changing the Engine’s source to a heavy degree).
EDIT: The GameInstance is also persistent between levels and keeps data so you cannot change the class of the GameInstance (if you’re talking about that)
I hope that I could help you 
Regards,