The character with the highest initiative acts first, the
second-highest goes next, and so on down the line. The
Storyteller handles initiative for each of his characters
too, and might decide for simplicity’s sake to have them
all act on the same initiative.
Ties are resolved by using the highest initiative rating;
if those are tied too, you could decide this tie in favor of the
character with the highest Wits or Dexterity (highest rating
wins). In story terms, both characters act simultaneously,
and the effects of their actions hit home at the same time.
Technically, you can roll a new initiative each turn.
To speed things along, though, you might decide to simply
have everyone roll one initiative for that particular combat
session and then stick with it for the rest of the scene.
The Storyteller must decide which option applies before
the action starts. Don’t try to use both options within the
same scene — it’ll get messy.