It's not separable. The stress all works together throughout the cycle. You're thinking about this as two separate days when, in reality, it's a week of stress. So to answer your question, the volume day and the intensity day drive the intensity day.
You also need to be able to actually lift heavy weights. It's not enough to just do high volume, light weight, even if the tonnage and resulting stress is super high.
So, it's not just about tonnage.
Does that answer your question?