Is it permissible to offer a perpetual charity (sadaqah jariyah) from one's personal money on behalf of a deceased grandfather, and is it permissible to include the deceased grandmother with him in the same perpetual charity?

There is no disagreement that the reward for charity reaches the deceased. Imam Al-Nawawi and others have reported a consensus on this, as narrated in Sahih Muslim from Abdullah ibn Al-Mubarak, who said, "There is no disagreement regarding charity." Whether the charity is given on behalf of one person or more, all of them will benefit from the reward, Allah willing. Therefore, there is no harm in making the perpetual charity (Sadaqa Jariyah) for the grandfather alone, or for him and the grandmother together.