To become a PhD Candidate, you must have successfully passed your qualifying exams. For double majors, this means BOTH exams. You are not admitted to candidacy for first one major, and then the other major - you must complete both exams and are then admitted to PhD Candidacy as a double major.
There are a number of fellowships, grants, etc. that are not available to you until you have been admitted to candidacy. For double majors, this means that you must successfully complete both exams before you may even apply for these opportunities.