It's easy, with Pianissimo's Stand-Alone version. Pianissimo features two tracks: the Left Hand track, and the Right Hand track. Now, you can record anything you want on these tracks, the names of the tracks are just there to help remind you what is on each track.
The Left Hand track button is located just to the left of the Record button. The Right Hand track button is located just to the right of the Record button.
To record a duet, first begin a new project by pressing File->New on the main menu. Then, press the Left Hand track button. It will turn white with a red glow around it. This means the Left Hand track is armed for recording. The metronome can make it much easier to line up your duet, so make sure the Metronome button is lit up; if it's not, press it to light it up. Now, press the Record button. Pianissimo will begin recording the Left Hand track. Play the first part of your duet now. When you're finished, press Stop.
Now, press the Right Hand track button. It will turn white with a red glow around it. This means the Right Hand track is armed for recording. Press the Record button again, and you'll hear the Left Hand part you've already recorded. Play the second part of your duet now. When you're finished, press Stop.
Now, press Play! You'll hear both parts of the duet performed together. If you want, you can re-record one or both of the parts. Just press the Left Hand or Right Hand button, depending on which track you'd like to re-record, and press the Record button again. Pianissimo will ask you to make sure it's OK to erase your previous recording. Then, make a new recording and press Stop when you're finished.
Remember, you can save your project as a MIDI File, and load it in later on, or you can save your recording as an audio file (such as MP3) to share it with your friends.