forwarding large files on servers
Yes.
What would be the problem exactly?
Personally I would recommend setting up an Nginx server at both ends to serve the file from the filesystem. Using Flask for hosting files is a waste of resources.
I have two Flask servers A and B. My user is on machine C. A is the public interface machine. The user wants to retrieve a 500GB octet-stream from B, but only the public interface on A is exposed. 
So I'd like something where:
User on machine C sends GET request to server A, request gets routed to server B, server B replies to C with the 500GB octet-stream without going through A. 
Is this possible?