Generally speaking, you do NOT want to parallelize disk I/O. Hard disks do not like random I/O because they have to continuously seek around to get to the data. Assuming you’re not using RAID, and you’re using hard drives as opposed to some solid state memory, you will see a severe performance degradation if you parallelize I/O(even when using technologies like those, you can still see some performance degradation when doing lots of random I/O).
To answer your second question, there really isn’t a good way to seek to a certain line in a file; you can only explicitly seek to a byte offset using the read function(see this page for more details on how to use it.