I've seen this question come up several times since the game's release. And I've seen it argued both ways - so, really, bottom line is, it's unclear enough that it keeps coming up and people have enough reasons to give support in either direction.
Personally, I prefer it at declare your targets before making your attack rolls. I choose to interpret it this way because just reading top down in the power description, it is the target line and then the attack line. Also, using an example, in PHB2 the shaman has a power (don't have the book handy, but it is a low level shaman attack power) that is also two attacks BUT after the target and attack line, it then has an effect line that says repeat the attack on the same or a different target. By extension of that, if you could choose targets per attack roll, then that effect line is entirely unnecessary and they could have kept the wording the same as the ranger twin strike wording.
Having said that, yes, I realize other people read it other ways and they have just as much support for other interpretations. So, my point being -- come up with a table decision for your group and just be consistent with it.