The mortal remains of a martyr should be gathered in a separate grave of his own unless necessity warrants burying more than one person in a single grave. In such a case, there is nothing wrong in burying a number of men in a single tomb, and preferably the one who is more religiously committed should be buried close to the Qiblah.
Sheikh Muhammad Saleh Al-Munajjid, a prominent Saudi Islamic lecturer and author, states the following:
The mortal remains of the martyrs are like those of other people. The mortal remains of each martyr should be buried in a separate grave, unless it is too difficult to bury each one on his own due to having too many corpses, especially in case of an overwhelming epidemic, killing etc… In this case, there is nothing wrong in burying two or three in one grave, and the one who was best in religious commitment should be placed closest to the Qiblah, as the Prophet (peace and blessings be upon him) did with those who were killed at the battle of Uhud.