Couldn't Windows Sound Recorder do that? You wanted simple, that's about as simple as it gets. Record the two separately, then mix one with the other using the "Mix with File..." option.
Does it have to involve a computer? You could get two tape recorders, a cord-spliter-thingy, and a length of cord. Connect the spliter to the microphone port on Tape Recorder #1, one end of the spliter to a microphone, the other end to the headphone port on Tape Recorder #2, press Record on #1, Play on #2, and speak into the microphone (you might want another spliter so you can listen to #2 play on headphones while you narrate).