Two things: first, the OP asked specifically about 2 processes; you are absolutely correct that more than 2 is probably better, but the exact number should be arrived at through careful profiling and testing.

Second, it is an excellent point that multithreading may be premature optimization: if one's execution speed is bound by some resource other than processor time (e.g. I/O, RAM, network throughput, etc.), then one might actually slow things down by multithreading. Again, profiling and testing are important.

Thank you for pointing these out, as I'd clearly forgotten to do so.