I'm a Nana myself, it's what my kids called my mother, and they had no other grandmother.
So, my grandchildren call me Nana,and one set, two sisters, call their other grandmother, "Granny", as their mother instructed them to, as it seemed the most "elderly" sounding-the daughter was having a failure-to-see-eye-to-eye time with her mother just then, though doing fine now.
The other set, two brothers, call their maternal grandmother "Memair", which is common. (Grandfathers are called "Pepair").