Multiple collections or single one containing everything

We need to store data for two different operations: charge and refund. These have a one to many link so one charge can have 0 or more refunds. We also need to be able to access charges and refunds by their own unique id.

Is it best to store these in separate collections and create a connection between a charge and its associated refunds, or is it best to store all refunds in the charge collection? If we store them in the charge then is it possible to access a refund if you only have the refund id. You'd essentially be querying for the charge that contains the refund with id=xx