For certain sports (running, rowing, speed-skating), you have to break your training into doubles
or triples otherwise you wouldn’t be able to get enough volume. Whether doubles/triples give you
other (unintentional at the time) benefits, that’s another story.
We are mainly focusing on metabolic aspects of training (lactate, burning of CHO/fat, etc.), but there’s whole other aspect of training that is never mentioned: hormonal and enzymatic adaptations.
A great book on this topic: