Sometimes you have a scent which is maybe too flowerly or too soft, sweet, fruity, feminin. or it is great but lasts really short. And then you don't wear them at all. Big mistake!
you can just first spray this linear or boring Perfume. when it's dry maybe few hours later you spray the other scent over it.
Or on the left side on your face or neck your Spray one Perfum and on the other one the other
for example youtr girlfriend has a sweet Vanilla flowerly Parfum and you have a linear Woody Parfum. You can perfectly combine and will get the total in new Parfum
It's a very interesting topic but 99% is all perfume users don't layer their scents !