How to offer the funeral prayer over more than one grave
Question

                How should a person pray who wants to offer the funeral prayer over more than one grave?.
Praise be to Allah.
    With regard to offering prayers over graves, one of two 
    scenarios must apply: 

    1.Either the graves are in front 
    of the worshipper, in the direction of the qiblah in one line, in which case 
    it is valid for him to offer one prayer for the occupants of all of these 
    graves;

    2.or the graves are scattered, 
    with some of them on his right and some on his left, in which case each 
    grave has its own ruling, so he should offer the prayer at each grave on its 
    own.

    Shaykh Muhammad ibn ‘Uthaymeen (may Allaah have mercy on him) 
    was asked: There are two janaazahs (funeral biers) next one another in the 
    graveyard: how should the prayer be offered for them after burial? Should 
    the funeral prayer be offered for each janaazah on its own, or can the 
    intention be made to offer the prayer for both? He replied: If the graves 
    are both in front of the worshipper, then he may offer one prayer for both, 
    but if each one is in a different place, then he should offer the prayer for 
    each one (separately). End quote. 

    Majmoo’ Fataawa Ibn ‘Uthaymeen 
    (17/159) 

    The one who is offering the funeral prayer over a grave 
    should stand with the grave in front of him like the bier is in front of the 
    worshippers during the funeral prayer. If the deceased was male, he should 
    stand at his head, and if the deceased was female, then he should stand at 
    her middle. 

    And Allaah knows best.