well, considering the distinct effect of the card, they would have to be sequential- and the impact would be less noticeable that way. so basically 40% chance to miss from the shield and then if that fails to stop it, 20% chance for the sword. this would end up being a lower % chance of blocked attacks over time, mathematically.
instead of 60 (40+20%) out of 100 blocked, it would be 40/100 and then + 20% of 60, which is 12, so 52%. if you wanted to make a duo with darkness, it would be 50/100 + 20% of 50, which is 10, so 60% net. so in practice the block will top out at 20% (if you have no shield), scaled down to 12% with fog shield (most likely combination), down to 0% (if there is a dim shield or some other 100% chance to block all damage shield)
hardly a game-breaking combination, especially 1) as a duo and 2) after thinking about it, it is like a single specific shield taking up the weapon slot, rather than the "dual shield" card which allowed any two shields to be used together (which is a good card in its own right, but i think this limitation to a single defensive weapon is a more balanced approach to a similar idea)