If there are any assets in the mother's sole name which require a grant, then you need two. For eg if there is real property involved, like a house or land in her sole name. The title to any property going from your mother to father, then to children, thus two grants are necessary.
But if the only assets, as you say, were in joint names ( as joint tenants) between mother and father, or in father's sole name, then you should only need one grant. So it depends.