This is something I already addressed in DawDreamer. In VST3 instruments that have secondary inputs, myPlugin->processBlock must receive a buffer that has 4 channels (a stereo pair for each of the two inputs). For example, the second input can be the signal for sidechain compression. You can't pass 2 channels. Instead you'd just put zeros in the third and fourth channels.
This is something I already addressed in DawDreamer. In VST3 instruments that have secondary inputs,
myPlugin->processBlockmust receive a buffer that has 4 channels (a stereo pair for each of the two inputs). For example, the second input can be the signal for sidechain compression. You can't pass 2 channels. Instead you'd just put zeros in the third and fourth channels.