Does training with two teams help or hurt development?

Occasional combined sessions can be useful, but 26 players only helps if the session design still gives them enough football. If combining the groups lets you create two or three good games with coaches working separately, it can be excellent. Players experience different teammates, different physical problems and more varied opposition. If all 26 are being coached as one group, the detail and repetition usually disappear. I would rather have twelve players in a purposeful 4v4/5v5 session than twenty-six waiting for turns in a huge activity. You can also combine only for part of the session: warm-up and larger games together, then split into smaller groups for the main work. At U13, smaller formats in training are completely valid even though 9v9 is the matchday maximum. Numbers should serve the learning, not dictate it.