I call them both Gran. My maternal grandmother would hate it if I called her granny or nan/nana because she thinks that would make her sound really old! My cousins sometimes call her grandma but to me, she's jut gran.
I used to call my late maternal grandmother :-
Avó (sounds similar to ah vaw) She had a Portuguese father. I only called her that when my late granddad wasn't around though, if he was around then I would call her as "poh poh" which is the Cantonese way of calling a maternal grandmother.
Paternal side, I was 6 when I first saw her, she was speaking to me in the vicinity of the granddad's house when my granddad chased her away with a pipe wrench and then to be told I must not go near her again. I didn't know the story then but much later I found out she had left the family when all their kids were rather young for some rich bloke. Well her children hasn't forgiven her and they don't exactly want me near her either..... mentioning her in front of my granddad would bring about the usual "I'd rather have died when my bomber crashed" whine. I guess if and when I do speak to her I'd address her as Frau *her married surname* as I don't think I could bring myself to refer her to anything around grandmother.
I'm from the midlands but my mum is from Jarrow (Newcastle basically but the other side of the river) and she always called her 'Nana Woods' (Woods is her maiden name). I think it's probably a colloquial thing.
I have one Granny and two Grandmas (stepfamily). And because that's what they told my mum they wanted to be called when I was born, so that's what they've always been known as. I think I'd like to be a Nana though.