Can more than one deceased person be buried in a single grave?

Under normal circumstances, only one deceased person is buried in a single grave. In cases of necessity, more than one deceased may be placed in the same grave. Indeed, the Prophet Muhammad (peace be upon him) implemented such a practice for the martyrs of Uhud. However, in this case, the bodies should be separated by soil or something similar. (Ibn Maze, al-Muhit, 2/193)

Unless the body previously buried has completely decomposed and turned to soil, it is not permissible to open the grave and bury a second deceased in it without necessity. After the body has decomposed and turned to soil, another deceased may be buried in the same grave. (Ibn Qudamah, al-Mughni, 2/381; Mawṣili, al-Ikhtiyar, 1/96; Nawawi, Rawda, 2/140; Mehmed Zihni, Ni‘met-i Islam, 602)

The time it takes for a body to decompose varies depending on the type of soil and the climate of the region. When there is a strong presumption that the previous body has decomposed, a second deceased may be buried in the same grave. If, upon opening the grave with the belief that the previous body has decomposed, some bones are found that have not yet decomposed, these bones should be moved to one side and a barrier of soil should be placed between them before burying the second deceased.

Source: T.C. Diyanet İşleri Başkanlığı, Din İşleri Yüksek Kurulu