I asked a similar question to customer service about this. You're gonna love their answer:
Question
I'm a little confused about when something is an attack, vs when it's an attack power. Here are a few questions that stem from this confusion:
If a Ranger is using Twin Strike, what is the order of events that happen for the attack?
Option A (following from the list bottom of page 269):
1. I choose to use Twin Strike
2. I choose two enemies for the attack.
3. I make two attack rolls.
4. I determine hit/miss.
5. I make two damage rolls
Option B:
0. I choose to use Twin Strike, which gives me 2 attacks.
1-5. I resolve these steps for the first attack
1-5. I resolve these steps for the second attack
If a Ranger is marked by enemy A, and decides to use Twin Strike to attacks enemies A and B, does he suffer the -2 penalty for being marked when attacking enemy B? What if he was instead using Split the Tree, does he suffer the penalty then?
If a Fighter is marked by enemy A, and decides to use Rain of Blows (assume with a flail), do all attacks generated from the power that don't target enemy A suffer the -2 penalty? Or is there no penalty as long as the primary target was enemy A?
If a Wizard is marked by enemy A, and decides to use Lightning Bolt, if the primary target is A, do the secondary attacks against different targets suffer the -2 penalty? If A is a secondary target, is the primary attack the only attack that suffers a -2 penalty?
I hope this wasn't too many questions all in one, but I'm trying to get a good picture of when something is an attack, and how it interacts with abilities that trigger on an attack, and the timing of it all.
Answer
All attack powers are attacks. Anything that involves and attack roll is an attack.
You can resolve your two attacks in either method depending on what you feel like doing.
When using Twin Strike or any other attack, if the character is attacking enemy A so there is no penalty.