It’s not quite as simple as this though in determining the fuel savings. On a heavier A320 you will have to considerably increase the thrust used on one engine to get the aircraft moving and burn potentially more fuel using just that one engine than if you had started both initially. Once you get going though and have the inertia it becomes an effective fuel saving initiative. Depending on the taxi routing I.e 90deg turns and reduction in speed required to navigate as such, especially at higher weights again the above is true. It’s not just a simple 50/50 saving.