Rewards are a valuable tool for potty training.  And reward charts can help give your toddler goals.  However with twins having reward charts can add a competitive edge to potty training.  Twins can potty train at different speeds, and it can be confusing for one twin to get a special prize and not the other.  Consistent rewards that they both have an equal chance to receive will help.  Keeping potty training a positive experience is key.