Fode Ballo-Toure was mainly brought in by Milan to act as the deputy for Theo Hernandez, who is the undisputed starter on the left. However, there is one solution that would allow them to play together.
According to Gazzetta Dello Sport, Stefano Pioli is studying several formation options ahead of the season as he wants to be more flexible. And one of the ideas that he is evaluating is playing Ballo-Toure and Hernandez together.
The 4-2-3-1 formation is still the primary one, there is no doubt about it, but with two competent strikers the manager could switch to a 4-4-2. And with that change, Hernandez could also be moved further up the pitch to left-mid.
This would allow Pioli to field Ballo-Toure at left-back, giving more freedom to Hernandez attacking-wise as he wouldn’t have to track back as much. Of course, this is an idea that might be useful for all the games, but might work for some.