The idea of receiving a gift and giving it to someone else is not generally considered good sportsmanship in normal relationships. In business, especially when it comes to promotional products, the more mileage that item gets the better.
In general, you don’t have to do much to encourage regifting of a promotional item. It happens naturally. In some cases, however, giving away two of an item with instructions to “share the wealth” is an easy way to double your advertising impact with little effort on your part.