Can an instructor sign off for my check ride if I didn't fly with him since a year?
No.  Instructor A needs to provide at least three hours of training in preparation of the practical test within two calendar months of the checkride.


  §61.39   Prerequisites for practical tests.
  
  (6) Have an endorsement, if required by this part, in the applicant's logbook or training record that has been signed by an authorized instructor who certifies that the applicant—
  
  
    (i) Has received and logged training time within 2 calendar months preceding the month of application in preparation for the practical test;
    
    (ii) Is prepared for the required practical test; and
    
    (iii) Has demonstrated satisfactory knowledge of the subject areas in which the applicant was deficient on the airman knowledge test;
  





  §61.109   Aeronautical experience.
  
  (a)  For an airplane single-engine rating...
  
  
    (4) 3 hours of flight training with an authorized instructor in a single-engine airplane in preparation for the practical test, which must have been performed within the preceding 2 calendar months from the month of the test
  





  §61.103   Eligibility requirements: General.
  
  
    (f) Receive flight training and a logbook endorsement from an authorized instructor who:
    
    (1) Conducted the training in the areas of operation listed in §61.107(b) of this part that apply to the aircraft rating sought; and
    
    (2) Certified that the person is prepared for the required practical test.
  




Instructor A has to provide the required training to the applicant before signing him off for the practical test.  I would agree it doesn't have to be within two calendar months... but three hours are required from an authorized instructor with two calendar months.  However, I would not, under any circumstance, sign off an applicant without at least three hours of training within the preceeding two calendar months.  How else would I know they are prepared for the practical test?