That'll depend to a large extent on your filesystem, and whether your bottleneck is in the IO of reading the files or in the parsing once lines are in memory. A bunch of threads all trying to read different files at the same time could possibly slow things down, if the disks have to keep jumping around from file to file to serve the different threads in turn. In general, it's probably faster to ask a disk for file1 and then file2 than to ask for both simultaneously. Filesystems have gotten pretty smart about such things, but ultimately the hardware can only do one thing at a time. If you have multiple disks, perhaps in RAID or a mirroring situation, then it may be possible to read more than one file at a time, and you could gain something. And if your parsing is complicated enough that you can parse a chunk from file1 while a chunk from file2 is being found and read, you could gain a lot.

Of course, the only way to find out for sure will be to try it.