Here is a simple patch to illustrate emulation of duophonic operation using the normally monophonic Solo Synth. It is able to play two pitches at once from the keyboard using oscillators Syn1 and Syn2. If you press one key both Syn1 and Syn2 play the key's pitch, as expected. If you press two keys the first one that is detected will play Syn1 and the second one will play Syn2. If you press additional keys Syn2 will play the pitch of the last key pressed while the pitch of Syn1 will not change.