My grandmas were addressed by Grandma >insert last name<
My kid's Grandmothers are called Nana and Granny (these have almost become a first name....grandchildren born for the last 30+ years have been calling them that)
Just so you don't hurt any feelings, how about explaining the confusion to the grandmothers and ask them how they would like to be addressed by their grandchild?
I would think that calling one
"Grandma" and the other "Grandma >insert name<" could even be taken as
the "first" grandma and the "second" one....just my opinion and it may not set right with second grandma...ok, I'm worrying too much