Alternate POA

I very much doubt that your father can delegate his POA for your grandfather to you in any way.  This kind of agency relationship is very personal, and unless the written POA itself allows this, a court won't uphold it.

What I could suggest is that your father prepare a POA for himself, to you, to be used when and if he became unable to care for himself, and he could add to the usual form a sentence asking you, upon the POA coming into effect with his disability, to apply for guardianship of grandfather, with perhaps a very short background explanation.  That wouldn't be legally binding, on you or on the court, but it would be something you could provide as evidence, when you did apply for the guardianship.  That guardianship, once granted, would give you the same ability to care for grandfather as a POA.

Prior advice is very sound.  A well-written POA form usually permits the appointment of another agent by the existing agent, so that bears looking at.  The referemce to a guardianship is good advice, a POA accomplishes lots of good things, but it can't cover every contingency.