When two sources of information have independent implications for control, e.g., pilot controlling pitch and roll of a plane.
When two sources of information imply the same action you get redundancy gain.
Two are negative
When the channel capacity is exceeded, e.g., reading two words that are very close together in space.
You want to process only part of the information and the implications of the second source is similar but incompatible to the first source, e.g. the Stroop Effect.