Forum:Mystical Cards of Light question

I have 2 Mystical Cards of Light on the field, so I have to select 2 S/T zones that cannot be used. My question is: Can I select the same zone twice? I read Card Tips here, and it says "as the Herald can ensure that you can still counter your opponent's moves despite the fact that you lack four of your five Spell & Trap Card Zones" But Why should I lock 2 zones, and why can't select the same zone twice?