Question

Can sendfile() linux syscall be used to send part of file from one mmaped file to other mmaped file?

Answer 1

From sendfile(2):

Presently (Linux 2.6.9): in_fd, must correspond to a file which
supports mmap()-like operations (i.e., it cannot be a socket);
and out_fd must refer to a socket.

So it looks like the answer is no.

Edit

I think I understand what you are looking for now. Check out splice(2).