A plate is Level IV, which is as high as it goes. I don't think you get any additional protection by doubling up plates.
From what I understand, the soft armor (Level III-A) is what stops the bullet. The plate (Level IV) is what stops the kinetic energy. Used together, it's a complete system and you're good-to-go.
If you only use a plate, bullet fragments (or the bullet itself) could bounce up into your neck/face/etc.
Dave
Co-Founder & Global Moderator
Delaware Open Carry
"You have to be prepared to do the violence in order to survive the violence."