No question is stupid.

Well, Shel and I are using different names, kind of like you do with Nana and Grandma (I'm Mumma, and Shel's Mummy..... we are expecting it'll probably turn into 'ma' and 'me' but we'll just see how it works). Because we each wanted something different so theres no confusion when we're called.

I know of people who both use mum, and I think thats really confusing, just like I think calling both grandmothers nana is confusing (my SIL does that and even I get confused at who she's talking about lol). I don't really know why some people use mum for both, though sometimes it's just that that's what their child decided to call them IYKWIM.

Basically, I can't answer you're question because Shel and I both wonder that as well... we don't want to be called the same thing anyway (mainly because I want to be able to go "you're turn, she's calling you!" ).

Hope someone can answer your question!