[Anton]

Well, even as old a game as X-COM did have a methood for deciding when a reaction can happen. It XCOM a unit would reacct only if a certain function depending on his Time Units was greater than a similar function for the target. As the result, the reacting unit wasn't always able to shoot first. Althoght the above method is absurd, it was there, and sometimes you could move around in an enemy's LOS before he started to shoot.

The easiest and most obvious technique could be to calculate the times needed to aim and take a shot for both units (using the units's specs as arguments: speed, turret speed, aiming mechanism...) and conclude the sequence of shots.

P.S.: Do you mean that being in the passive phase guaranties for a unit to be the first to shoot at anyone who enters his LOS, if both units spot each other simultaneously?

P.P.S: Thanks for the reply!