Using more than one thread to read a file is usually a really bad idea. However, if the contents of the file were designed for it, you can do it.
The file requires an index of sorts that says which records can be found in what position in the file. Read the index to find out what records the file contains, and divide them up over separate tasks that are responsible for processing a portion of the records.
Why do you want to do this?
The mind is a strange and wonderful thing. I'm not sure that it will ever be able to figure itself out, everything else, maybe. From the atom to the universe, everything, except itself.