How to add multiple fields
the easiest way is field_collection:


  Provides a field-collection field, to which any number of fields can
  be attached.
  
  A field collection is internally represented as an entity, which is
  embedded in the host entity. Thus, if desired field collections may be
  viewed and edited separately too.


You can make a compound field in code as this Phase2 Blog Post describes .... maybe there's a module for a compound field, but its usually not worth the effort.