It's not shield in front VS shield in back... it's shield in hands VS shield in back.
Try Mount and Blade if you don't understand why it really matter.
I believe i do ...
I simply dislike this game trend that shield on back dont protect anything at all. :-/
You know what i mean right?
You wear a robe ... so your back is protected with one layer of common cloth ...
You wear a shield on your back ... so your back is protected exactly the same.

Its odd.
On top of that AC is not a number showing how efficient your armor is. Dexterity also increase your AC.
True that ...