Burying two Bodies in one Grave

The great Hanafi jurist (faqih), Imam Ibn Abidin (Allah have mercy on him) explains this very issue of “burying more than one body in the same grave”, in his brilliant supercommentary (hashiya) on Imam al-Haskafi’s Durr al-Mukhtar. The following is the Shariah ruling primarily based on Ibn Abidin’s explanation in his Radd al-Muhtar:
It is impermissible to bury two bodies together in one grave unless due to necessity (such as the shortage of graves, or a natural disaster causing many people to die, etc). It will also not be permissible to dig up a grave in order to bury another body unless one is certain that the earlier body has disintegrated, decomposed and transformed into soil.

If upon opening a grave, some bones are discovered, then they should be assembled and placed to one side and then the other body may be buried. One should make a barrier with the soil between the bones and the new body.

To bury more than one body in the same grave with the intention of burying the deceased with his relative, or due to the shortage of graves in that particular graveyard will not be considered a genuine need, even if the graveyard is a virtuous one, where pious people are buried, as long as another graveyard is available. This impermissibility is even more in the case of digging up a grave where a body has already been buried, for burying other bodies in the grave of one whose body has not as yet transformed into soil is dishonouring the sanctity of the first body. (See: Radd al-Muhtar ala al-Durr, 2/233)

Therefore, burying two bodies at once in one grave would be unlawful, and if one did make such a will, it will not be fulfilled. Similarly, to bury a second body in a grave where the previous body had not yet disintegrated or transformed into soil would also not be allowed. Thus, one should be certain of the body being transformed before intending to open up a grave. The duration of this would depend on the place and area one is in, and could be inquired from the experts.

With the above, the ruling with regards to your specific question becomes clear, in that if the will made by the deceased to be buried in the same grave as his/her spouse was such, that if it was implemented, it would entail opening up a grave in which the body of the earlier deceased was not transformed into soil, then this would be impermissible. However, if it was after a long period, in that one was certain of the earlier body being transformed into soil, then it would be permissible.

And Allah knows best