I follow tennis but havent seen the rules of such schedule conflicts. My best guess is that Singles take precedence and they have a minimum time between matches for players.
As for stamina, these top players practice for hours before their actual matches so it shouldnt be an issue with back to back matches. As long as JP gets time for a change, comfort break and nutrition+hydration, she should be fine to play back to back matches. Plus a doubles game doesnt sap the energy as much as a singles game.
On a related note, it was another fantastic match between Nadal and Djokovic today. I just am in awe of these guys who play so hard for 4+ hours and can still sprint about the court.