Is it permissible to bury the deceased in the grave of one of his parents? And if permissible, what is the required time interval between the two deaths?

It is not permissible to exhume a grave or to bury another deceased person in it as long as there is any trace of the deceased, for the grave is exclusively reserved for them. This is not permissible even if it was the will of the deceased.

However, if no trace of the deceased remains, and their body and bones have disintegrated and turned into dust, then it is permissible to exhume the grave and bury someone else in it. No specific time limit can be set for this, due to the variation in locations; rather, it depends on the opinion of experts.