i agree... it's up to the grandparents to be to decide what they like to be called, and not the parents unfortunately! I can't see why there can't be two "pa's" tho

I have a nana and pop and both sides and we tell the difference by calling them their nick name with it... eg, my mums mum is nana merilla XD
Maybe you could suggest the same? You could have a Pa John or something...

Jackson has the same. Then my parents are Nan (or Nan-G XD) and Gop (Grumpy Old Pop - it's a bit of a running joke... because my dad thought he was too young to be a pop, however he adores DS now!)
DH's side, his great grandad is "pop pop" and DH's parents are just nana and pop...

good luck! but I'd say just leave it to the grandparents!