When holding two shields at the same time you can make yourself pretty much invinsible, but even when the shield in the off-hand is used it will not take damage when the shield in the main hand is used at the same time.
MC-86655
When in testudo formation with shield only the shield in main hand takes damage
Resolved
Duplicate
0
karstvgl
(Unassigned)
Unconfirmed
Minecraft 15w34a
Retrieved
The second shield doesn't block (only visually) therefore no damage.