I know in my situation, I get along extremely well with my mil, as well as my mom. I also strive very, very hard to make sure things are fair between them all, etc.
So in the scenario you described, I would let the other grandma have an opportunity to go in on the new, more expensive scooter & would stress to gs that it is a gift from his 2 grandmas.
That is assuming she would want to do that. I know my mil & my mom would, if either was in the other grandma's shoes. And honestly, just knowing my situation, if my mil or my mom was in the other grandma's shoes & no offer to go in on the new, more expensive scooter was made, there might be some hurt feelings.
Just a thought.