i had two grandmas and two grandads growing up - differentation was by surname when necessary.

we now have two grandads a grandma and a nanna (who hates nanny cause it makes her think the kids are calling her a goat - which is amusing cause guess what all mine have called her initially (due to speech ability) - nanny. No confusions even from a young age although sometimes they don't use the surname for the grandads they just say nannas grandad or grandmas grandad if they need to differentiate

its not something I was precious about it so it didn't worry me all that much. it is obviously important to both of you for different (rightly or wrongly) reasons so some more calm discussion is probably in order. Ours were chosen by the grandparents themselves (and in turn the children in their ability to say them) and I'm of the opinion unless there are really good reasons this is appropriate. Obviously two of the same name (in your case Pa) I don't see a problem or that they need to "match" We do have all the same names throughout each family though so they are all called the smae thing by each grandchild.