Bukhari

Hadith Commentary:
Earlier in the hadith, it was mentioned that Jabir radi Allahu anhu opened his noble father's grave six months after burial and transferred him to another place. In another narration, it is stated that the graves of his father, Abdullah bin Amr, and Amr bin Jamuh radi Allahu anhum, were disturbed by a flood, while both were in the same grave. When the grave was opened to transfer them, they were not affected in the least; even Abdullah bin Amr had a wound on his body, and at the time of his death, his hand was on that wound, and he had been buried in that very state.

When he was taken out of the grave and his hand was moved from that spot and then released, it returned by itself to its original place. It appeared as if they had died only yesterday, even though there was a gap of forty-six years between the incident of Uhud and the opening of the grave.
(al-Muwatta’ of Imam Malik, Kitab al-Jihad, Hadith: 1044)

Both narrations mention them being in the same grave, so the reconciliation is that Jabir radi Allahu anhu must have opened the grave of his noble father and Amr bin Jamuh radi Allahu anhu twice: once six months later, most likely after seeking permission from the Messenger of Allah sallallahu alayhi wa sallam, as at that time the Messenger of Allah sallallahu alayhi wa sallam was alive.

After that, their graves were placed close together, because there was a deep connection between the two. Then, a flood or similar event disturbed their graves and made them into one.

Later, when Amir Muawiyah radi Allahu anhu caused a spring of water to flow at that place, Jabir radi Allahu anhu again became concerned that their bodies might be swept away by the water. For this reason, after a long period, he again opened the grave and saw that both bodies were still in their original state.
(Fath al-Bari: 3/276)