The two players can also play online while using split screen. This game mode is normally exclusive to the Xbox 360 version of Left 4 Dead and Left 4 Dead 2, but may be accessed in the PC versions if you have a controller for the second player and modify some files.
Serial memory processing - Wikipedia. Serial memory processing is the act of attending to and processing one item at a time. This is usually contrasted against parallel memory processing, which is the act of attending to and processing all items simultaneously. https://en.wikipedia.org/wiki/Serial_memory_processing
It is quite common to find somebody asserts that the brain is a 'massively parallel' system. This assertion is simply false. In the area of information processing, 'parallel system' means a system that contains more than one processors that work independently.