♦According to PHB 269, attack resolution is following:
Making an Attack
All attacks follow same basic process:
1. Choose the attack you'll use. Each attack has an attack type.
2. Choose targets for the attack (page 272). Each target must be within range (page 273). Check whether you can see and target your enemies (page 273).
3. Make an attack roll (page 273)
4. Compare your attack roll to target's defense (page 274) to determine whether you hit or miss.
5. Deal damage and apply other effects (page 276)
Emphasis is mine.
Going by this, RAW seems clear.
You'll have to first choose one or two
both targets for Twin Strike
(targets can be same creature) before doing attack rolls.
It's possible to interpret "1. Choose the attack you'll use. Each attack has an attack type." as meaning
Attack: line of the power, but IMHO it's pretty obvious they meant "attack power" simply because the other interpretation will make it impossible to do proper resolution of power as the
Attack: line includes attack roll.
I think they would have included something about
Attack: line being considered before
Targets: line at PHB2 218 if designers had actually meant it. But I might be wrong.
But Samir's post at
http://www.enworld.org/forum/d-d-4th-edition-rules/264012-twin-strike-question.html#post4908010 uses exactly same rules as I here to argue completely different position, so it's obviously not as clear cut case as I have thought beforehand.
WOTC should really use more exact language, it's annoying that they use "attack" interchangeably with :
* Attack power / Utility power that has
Attack: line
*
Attack: line in powers
* Attack roll
Never specifying what they actually meant.
Edit:
I remembered Twin Strike wording incorrectly, corrected my text to reflect this.
Twin Strike:
Targets: One or two creatures
Attack: Strength vs. AC (melee; main weapon and off-hand weapon) or Dexterity vs. AC (ranged), two attacks