Ok, I'm sure I'm just being an idiot, and I'm sure I read the answer to this in one of the rulebooks but now I can't find it.
Between every 'combat turn' (that is to say, exhausting a ready unit to make an attack or passing if you are unable to do so) there is an Action Window. Presumably both players may take actions during each window, and presumably the player with Initiative (for the combat) gets to act first?
EDIT: Ignore me, I found it again.
Rules p.20 wrote:
When an action window opens, the player with the initiative has the first opportunity to initiate an action, or pass. Opportunities to initiate actions alternate between the players until both players consecutively pass, at which point the action window closes.