Short Answer

It is not allowed to bury two bodies in one grave. This can only be allowed in the case of necessity where there is no other nearby place to bury a body or in the event of a natural disaster.

What the Scholars Said

Imam Kasani said: If there is a genuine necessity then it can be allowed. [al-Badāiʿ aṣ-Ṣanāiʿ 2:357]

Imam Ibn Abidin said: It is not allowed to dig up an occupied grave to bury another body until one is convinced that the first body has decomposed into soil. This would be dishonoring the first body. [Radd ul-Muḥtār 3:138]

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Shaykh Mustafa Umar