If you are allowed to have two Initiative cards (usually only special monsters) then you have two fast actions and two slow actions. The monster acts and each initiative card "refreshes on its own initiative(see edit below). When it comes to reactive actions, I usually use the the closest initiative action available. So if a monster has initiative 5 and 8 and is attacked during initiative 3, I use the fast action on the initiative 5 (the monster will only have a its slow action left there). If the monster is attacked again during initiative 4, I use the fast action from the monsters initiative 8 card. If we assume the monster is attacked twice during initiative 4 (by a guy with two weapons for example) then I will use the other action on initiative 3, leaving only one action for the monster (a slow on initiative 8).
edit: now when I thinking about it, actions are of course refreshed at the start of a round before anyone has done anything. In MY0 though this approach will not work since there you "borrow" you next action (slow action in FbL) when you parry (it is a different melee system in MY0).
“I’d say thistles, but nobody listens to me, anyway.” - Eeyore