I disagree. That's the sequence for a single attack. Twin Strike says quite plainly in the power description that it's two attacks. You would follow that full sequence for the first attack, and after it's resolved start over from step 1 for the second attack.
It says choose "targets" in step 2.
Also:
Subject: Powers that allow one or more targets
Discussion Thread
Response (Support Agent) 10/13/2008 12:36 PM
Hi Caliban,
Thank you for contacting us. When using powers that allow you to target ne or more creatures, you must choose your targets before you make your first attack roll. You cannot wait to see if your first attack drops the target before choosing the target for the second attack.
I hope this information is useful. Please let me know if you need anymore help!
Good Gaming!
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Marc
Online Response Crew
Wizards of the Coast
Customer (Caliban) 10/13/2008 12:16 PM
If a power (such as the Ranger Twin Strike at-will) has a Target of "One or Two creatures", do you have to choose the number of targets before you make your attack roll?
Can you wait to see if your first attack drops the target before choosing the target for the second attack?
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Book Name: PHB