Anna, I imagine that it is different in various parts of the English speaking world, but in English speaking Canada you can use the family name to distinguish them when you are talking about them. My mother's mother is my grandmother. She would be Grandma Smith and her husband Grandpa Smith. My father's mother is also my grandmother. She would be Grandma Jones and her husband Grandpa Jones. When we are talking to them in person, we just say Grandma or Grandpa. If you have them all together in the same room then of course you could use their last names or their first names to adress them. Grandma Mary or Grandma Smith, Grandpa Henry, etc.

mother's mother/ father's mother = aachchi or aachchi amma mother's father / father's father = siiya or aatha

In my language you can say "Avó" for grandmother and "Avô" for grandfather. The write seems very similar, but the sound is completely different. Now, to distinguish between mother's father and father's father in any language, I think you have to use the first name or the family´s name after grandma and grandfa.

My grandmother on my mother's side. (Mother's side = mother's side of the family) ...etc. There are of course other ways to address them. My kid calls my mother Grandma, calls my stepmother Noonna, and calls her mother's mother Obaachan. Go figure!

Hm, the question is resolved and thus closed, so how can we provide input on other languages? pity... the system is apparently not flexible enough...