Math would rather suggest that two shields give a chance of 1 - ((1 - 0.3)²) = 51% for blocking with at least one shield. Not sure how the actual damage blocked would look like if both shields block (9% chance).
They stack the same way they do in dota. It's not a pure, additive stack. Two shields does not give a 30 + 30 = 60% chance. Each shield independently has a chance to block the damage. So getting a second shield DOES increase your chance to block the damage, but its not as simple as adding the two chances to block together. This is also true of crits, bashes, etc.