Dependent envelope custom picklist fields to create 2-level classification hierarchy

We’d like to have 2 envelope custom fields (Document Type and Document Sub-Type) to create a 2-level classification hierarchy for our envelopes. We’d like the values available in the Document Sub-Type field to be dependent on which value was selected for Document Type. i.e. the options available in the second field should be dependent on which option was selected in the first field.

This functionality would mirror what we have in Salesforce for classifying documents (dependent picklist fields), and we would like to map the Envelope Custom Fields to the corresponding fields in Salesforce. We would like users to be able to select valid combinations of Type and Sub-Type when sending an envelope via Docusign, and to have these updated back into Salesforce, so that documents sent via Docusign are classified/categorised according to the same schema as those which are sent from inside Salesforce.

We would also like to be able to control the order in which these fields are displayed when sending an envelope. At the moment they are displayed in alphabetical order, which means the user thinks they are supposed to select a Document Sub-Type before they select a Document Type. We would like to be able to show these fields in the correct order, especially if the values in the second field are dependent on the value selected in the first field.