I think it's more common to call them Grandma and Grandpa Lastname or Grandma Firstname and Grandpa Firstname. I've also seen it where one set of grandparents are Grandma and Grandpa and the other set is Nana and Papa or Mimi and Pop Pop or whatever set of less formal terms they use for the relations.

I'm not sure I've met anyone who doesn't have more familiar terms than dad's mom and mom's mom. They're probably out there, but not super common.