It is not strictly correct that between is used for two things and among for more than two.
When exactly two entities are specified, between should always be used:
“This contract is entered into between the Seller and the Purchaser.”
However, when more than two entities are involved or when the number of entities is unspecified, the word choice depends on what you want to say. Between should be used where the relationship is distinctly one-to-one:
“The agreement was entered into between the Seller, the Purchaser and the Guarantor.”
Among should be used where the entities are considered as a group, mass or collectivity: “There is consensus among shareholders that this approach be adopted.”