If two Mystic Barriers are on the battlefield with two different chosen directions, you may not attack a player or planeswalker unless that player or the controller of that planeswalker is the opponent seated nearest you in both directions. (This happens most often in two-player games.)
For example, if I cast Casualties of War targeting an Eldrazi Monument and a Runeclaw Bear, would the Bear end up getting destroyed because the ruling for Casualties of War states that they are destroyed in the order specified? Thus (since artifacts are listed first) Eldrazi Monument will get destroyed first and stop granting indestructibility to Runeclaw Bear in time for Casualties to destroy it?
Does the same hold true for Decimate?