I don't see why you can't have 2 pa's both my granparents were nanna and pop.

My poor girls are going to be so confused, my parents are seperate so we have Nanny & Poppy (mum & step dad), Poppy & Oma (dad & stepmum) and Nanna & Grandpa (pa).
Each of mine got to choose what they were called and it was based around what my nieces and nephews already called them.
My stepmum is dutch and my brothers daughter does not call her nan or oma. i did not think this was appropraite as my stepmum raised us and although she is not MUM to us she is a major part of our lives... i gave her the choice, you pick an aussie version (ie nanna, nanny etc) or Oma.. she picked Oma.

I say you let them pick their own, if there are two Pa's it means 2 people get to be excited when they say there name rather than your child having to learn more

GL with it hun...