Step one is choose your attack.
Step two is choosing targets for your attack.
Step three-five is rolling and resolving.
Now, 'choose your attack' is the part where you decide what power you're using and the mode you're using it in.... this means that the 'Making Attack Steps isn't just the part between Attack: and Hit:.
This means, therefore, once you've decided you're Twin Striking, you choose targets, then roll your attacks.
Compare to Sudden Strike, which has you make one attack, then shift and make another attack. If Twin Strike allowed you to resolve one attack before targeting the second, it would look like this:
Attack: Strength vs AC (main weapon) or Dexterity vs AC (ranged)
Hit: 1[W] (main weapon) or 1[W] (ranged)
Effect: Make a secondary attack against either this target, or another target in range.
Secondary Attack: Strength vs AC (off handed weapon) or Dexterity vs AC (ranged)
Secondary Hit: 1[W] (off handed weapon) or 1[W] (ranged)
Hunter's Quarry, however, -does- let you assign the damage after you've rolled both the attacks for the Attack line of Twin Strike, because it's part of the same attack.
Here's the order:
Step 1: Decide you're gonna Twin Strike
Step 2: Decide your target(s) for Twin Strike's attack.
Step 3: Roll your attack rolls for the Twin Strike's attack.
Step 4: Determine if they hit the AC of the target(s) At this point, you can decide to apply Hunter's Quarry damage
Step 5: Roll and apply damage.