The ability to remain in the present moment is the cornerstone of mental toughness.
Being in flow is achieved by focusing on playing in the present moment. Thinking about the past or focusing too far into the future can be the largest obstacle to execute in the present moment. If you practice two-shot combinations, you are automatically preparing for unpredictable circumstances that may come up and your response to them will be more calm. My experience is that when players learn about different combinations of two shots in a row and the objectives that come along with them, they actually enjoy playing and managing their strategy better.
The player who can simplify and automate decisions based on two-shot combinations can quickly get in flow and remain there for longer periods of time.
Focusing on just one ball at a time can lack a sense of challenge and focusing on three or more balls in a row can trigger the mind to think too far into the future. The goal is for a player to keep tasks simple enough to achieve them without jumping too far ahead or getting caught focusing on the past shot. After two in a row is achieved, players reset, looking for the next combination, focus on two more in a row, reset and so on…