That rule is not solely refering to 'Effect' lines. That's simply an example.
However, the PHB states that ranged attacks that effect multiple targets each have their own distinct attack and damage roll.
However, it does not mention that targets are selected in the same sequence. Therefore, while the rules for Melee and Ranged attacks supercede the rules for the resolution of steps 3, 4, and 5 of Making an Attack, it does not contradict step 2 and therefore does not supercede that.
I'm not sure there's any supercession at all. This isn't general vs. specific, this is two general rules that aren't even in contradiction.
The PHB states that (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.
Thus, Twin Strike is two separate attacks, each with its own attack roll and damage roll.
The PHB also tells us how to make 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. You choose your target at the start of each attack, not at the immediate moment you use the power.
If you chose your target at the immediate moment you use the power, then attack powers that involve movement prior to the attack would not work if you weren't in range to start with, even if the power could move you in range before you made the attack.
Further, step 2 is pretty explicit:
'Choose targets for the attack.'
Please do note the plural there.
I'm not sure this affects anything, RAW. You choose the targets for each of the two Twin Strike attacks when you are making each attack--of course, under normal circumstances, each attack only hits one target. The plural just seems to allow for the circumstance if each of the two attacks hits multiple targets.
CS responses in the past have favored the "choose targets one by one" interpretation, FWIW (which isn't much). I think this will be a good Twitter question for this Thursday.