I'd imagine kids that happen to grow up in households with two granddads or two grandmas would find solutions - I remember we called the latter two "grey grandma" and "white grandma" on the basis of their hair colour, though I'm not sure how often we used those names to their faces. And of course plenty of adults call their parents and step-parents or parents in law of the same sex the same thing. I'm curious if there are any languages that don't have separate words for "mum" and "dad" though. Tried googling but all the results were about how different languages use similar words for each term individually.