Be careful now
this might shock you
*Disclaimer*
The following picture is not rated for kids
*end Disclaimer*
http://tinyurl.com/y95j5o7
Now you know...
2 Iron Bucklers: 1 - (1-0.6)(1-0.6) = 1 - (0.4)(0.4) = 1 - 0.16 = 0.84 = 84% to block 20 damage; 0.84 x 20 = 16.8 average