I did this before also on some of out cars. We did this to the front driver and passenger windows as we wanted dark tint but needed the side mirrors to be seen properly. The purpose of the double tint was that we cut the second tint layer (top) at the part where the eyesight of the driver is to the sidemirrors. The result was you can see the mirrors well at night and yet from the outside, the tint still looks "buo" as you don't see the tint cut inside.