In 2002, Dallas musician and visual artist Paul Slocum developed a cartridge called Synthcart for the Atari 2600, which allows the user to turn an Atari 2600 into a two-voice synthesizer and drum machine. Source: Internet
The exception to this method was the conductus, a two-voice composition that was freely composed in its entirety. Source: Internet
These treatises provided examples of two-voice note-against-note embellishments of chants using parallel octaves, fifths, and fourths. Source: Internet
They are Leto leta concio (a two-voice round) and O quanto consilio (a four-voice round). Source: Internet