OR2v is a free Analog Duophonic Synthesizer VST Instrument.
Odsay is a two-oscillator analog synthesizer free VST instrument, and was the one of the first synthesizers with duophonic capabilities (the ability to play two notes at the same time).
The Moon Sono is a free VST 6 features two VCOs with changeable waveform (sawtooth, triangle, square, pulse), one low-pass VCF, a VCA, two multimode LFOs for modulation.
Micromoon is a free monophonic VST synthesizer instrument, featuring 1 variable waveshape VCO.
DavoSynth is a free vst synth instrument inspired by Davolisint (a rare Italian keyboard created in 1972). There are two oscillators called VCO1 and VCO2 – but there are no waveform selection, syncing options or envelopes.