Say you don’t want to be a lone pontificating hermit but instead want a cohost or want to interview another person. The easiest and best way is for each person to have their own mono microphone going to a separate recording channel. This makes it a lot easier to make edits and repairs. Be kind to your listeners and try to make both voices as close to the same volume level as you can. Let your ears be your guide more than the waveforms, because even when the waveform says both voices have equal volume, some people sound louder. (There’s that psychoacoustic thingie again.)
If you’re using a netbook or laptop with a real stereo microphone port, you can use a splitter for plugging in two mono mics.