Except for PHB2 saying that things happen in an explicit and specific order, I'd agree.
So, yes, it -was- ambiguous. And that ambiguity has since been solved by later rules. It's happened before (errata, Stealth), it'll happen again (MM rule on immunity to foo).
I think I know what part of PHB2 you are talking about, so let's look at it for a second.
Sequence: The order of information in a power description is a general guide to the sequence in which the power’s various effects occur. For example, an “Effect” entry might appear above attack information in a power description to indicate that something happens before you make the attack.
Underline is mine. I'm not sure I would call a "general guide to the sequence" an "explicit and specific order." It says it's the order in which the "power's various effects" occur. The
Target: line isn't one of the power's "various effects" any more than the Keywords are. It's a descriptive phrase, not an effect to be applied in a specific order.
The
Special: line always appears at the very end of the power, as corroborated by this section in PHB2. Does this mean that anything listed under Special isn't considered until after the Attacks and Effects are resolved? Of course not.
Now let's go back to PHB1 for a sec.
Pg. 270:
Targeted: Melee attacks target individuals. A melee
attack against multiple enemies consists of separate
attacks, each with its own attack roll and damage roll.
...
Targeted: Ranged attacks target individuals.
A ranged attack against multiple enemies consists of
separate attacks, each with its own attack roll and
damage roll.
So Twin Strike is two separate attacks, each with its own attack roll and damage roll. Now how do we make each of these attacks?
Pg. 269:
MAKING AN ATTACK
All attacks follow the 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 the target’s defense
(page 274) to determine whether you hit or miss.
5. Deal damage and apply other effects (page 276).
So for
each attack in Twin Strike, you choose a target, make an attack roll, and deal damage. There are two attacks, so you do this process twice. Hence, you choose your target at the start of each
attack, not at the immediate moment when you use the power.
IIRC, post PHB2 CS agreed with this interpretation.