This needs lots of AP and if possible some Friend Spheres but if you do it right your result will be a fast Auron and a strong Rikku or if you want you could just make any character like another. All you have to do is after you are done with one's sphere grid use a friend sphere to go to another one's grid, like for example when you get with Auron to Sentinel and Entrust you have theoretically finished Auron's grid so just use a Friend sphere to go to Rikku's or Tidus' grid and it would be even better if the character you are going to is close to the end so you would take it from top to bottom, I've done this and now my Auron is as fast as Rikku and casts BLK magic like Lulu. This is cool so try it out "ya".