Hadn't noticed that it became enabled since I assumed it only related to Unison mode.
Still not working as I expect, though:
With both oscillators set to "Classic", and the Sub slider turned up, the sub-oscillator is only heard through the left speaker, as expected. Also, the sub disappears completely if I set the Filter Balance slider fully right, again as expected.
However, as soon as I switch Oscillator 2 to Hypersaw (with Density at 1 and Detune at 0) I suddenly get a Sub coming from my right speaker as well. Now Osc 1 and Osc 2 sound essentially identical, with a single saw and a square wave an octave lower.
So, it would appear that each Hypersaw oscillator has a built-in sub, where as the Classic oscillators share a single sub-oscillator. This kind of makes sense when you think about it, since the Hypersaw sub follows the density and detune parameters of the master oscillator, which can be different for Osc 1 and Osc 2. However, the manual makes no mention of this. Even the diagram in the Virus Control GUI itself makes it appear as though there's only a single Sub, linked to Oscillator 1 in Split mode. Too bad there isn't a way to mix the oscillator-to-filter routing freely like on the Q. Also, two four-pole filters in parallell would have been cool, instead of being limited to 2-poles. Also, being able to control the stereo spread of each Hypersaw oscillator before the filter stage a la Sylenth1 would have been neat. But I guess each synth has its strengths and weaknesses