You need two peels because..

Well.. because you would need one peel to prepare the next pizza, portion of bread rolls or baguettes, and one peel to get the first out of the oven. It's as simple as that. 

With only one, you would need the peel to get the first portion out of the oven, and then prepare the next portion on the peel, before letting it glide onto The Pizza Steel.
It would work, surely, but you would waste time waiting - and who likes to waste time?

With 2 peels you will be able to speed up the process and your friends and family won't have to wait too long for that next pizza to arrive ;-).
I would certainly miss my second peel, so why should you make do with one?
Signature Mr Pizza Steel