Overlapping Auras: If auras overlap and impose penalties to the same roll or game statistic, a creature affected by the overlapping auras is subjected to the worst penalty; the penalties are not cumulative. For instance, if a creature is affected by three overlapping auras that each impose a -2 penalty to attack rolls, the creature takes a -2 penalty, not a -6 penalty.
Published in Monster Manual 2, page(s) 6, Rules Compendium, page(s) 308.