it has 2 output channel, but no known hardware using this feature for stereo sound.
later hardware paired this with external bankswitching logic, but its logic is not emulated now in current furnace revision.
Allumer one is just rebadged Seta's thing for use in their arcade hardwares.
It has 16 channels, and all channels can be switchable to PCM sample or wavetable playback mode.
Wavetable needs to paired with envelope, this feature is similar as AY PSG but it's shape are stored at RAM: it means it is user-definable.
In furnace, this chip is can be configurable for original arcade mono output or stereo output - its simulates early 'incorrect' emulation on some mono hardware but it is also based on the assumption that each channel is connected to each output.