How to show both currencies at once in the node page for price field
Commerce Multicurrency does exactly that. 


  This module enhances some of the multi-currency capabilities of Drupal
  Commerce.
  
  Current features:
  
  
  UI for fine granular definition of exchange rates.
  (Every currency combination can be defined separately if needed)
  Synchronization of currency exchange rates directly from the European Central Bank (ECB).
  UI to specify which rates shall be synchronized and which are handled manually.
  Generation of currency specific price fields inclusive generation of rule-set to handle them.
  hooks for easy integration of custom currency exchange rate sources.