How to offer the funeral prayer over more than one grave

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.