What you need is not GDevelop-specific.
- you need to know which player’s tun it is. Put that information somewhere, maybe a scene variable. Decide how you want to switch the turns (time triggered, event triggered) and modify that variable accordingly
- one player will hurt the other player if some event happens (keyboard, mouse, joystck, …).
Add another condition that it must be this player’s turn
- Probably you also want to add this condition to the player move functions, so the inactive player can neither attack nor flee