My grandparents are "grandma" and "grandpa". In my family, if we need to differentiate between my mom's parents & my dad's parents, we usually tack the applicable last name on the end. "Grandma" becomes "Grandma Orr", for example. I have, though, often seen other people refer to one set of grandparents one way ("grandma" & "grandpa") and the other set with a different nickname ("papa" and "meemaw" or whatever the hell you can think of).
I've never had two moms, so I don't know what I would call them. If I was raised by two moms, I would call them whatever they taught me to call them.
A much more common scenario would be where a husband & wife divorced and each got remarried, so a child has both parents and step-parents. And again, it would pretty much depend on the specifics of the family dynamic as to who would be called what. I've known people who are closer to one or both step-parents than they are to to their biological parents, and who thus call their step-parents "mom" and "dad".