I'm sure your mum would respect that, and love to be given that part in the wedding day.
I'm in a similar position in that my mum and dad split up when i was young and i don't have a relationship with my dad...although we were in touch in my late teenage years, he never made me feel equal to his new family. he found out we had set a date for the wedding and apparently was really angry they weren't invited...but i'm not having people at my day just because.
I'm having my mums partner as my "father of the bride", he was never really a father figure tbh, but he was and still is always there for me, more than my actual dad ever was.
I would love to have had my mum as my "father daughter dance", but unfortunately she is no longer with us. So i'd say do it! take the moment and make it what you want! share the "father of the bride" between the 2 of them!
xxxxx