I was referring to the errata'd version, which gives the secondary attack on an effect line after the primary attack (as do other powers such as Wildblood Frenzy).
If it's "two attacks," I read it is "declare both, execute both."
If it's on attack with "effect: make the same attack... again," I read it as sequential.
Except the rules really are quite clear, I don't know where there is any real ambiguity. The "resolving an attack" procedure is followed for an attack. Twin Strike has 2 attacks, you follow the procedure twice, once for each attack. Seems clear as day to me.
I think people get confused between an
attack power which grants attacks, and the individual attacks which are granted, which follow the procedure. Where there could be ambiguity, powers which are area attacks, the rules clearly state a different procedure where you roll to-hit for each target and then damage is dealt once. Admittedly this is not spelled out with a separate attack procedure rules block, but the differences are clearly indicated in the rules describing close and area attack types and it is pretty easy to understand that in those cases its one attack and you need to simply iterate the "roll to hit" step as many times as needed before going on to the following steps.
There is a lot of potential variation in powers too, so the procedure has to be interpreted in a slightly flexible manner. Supposing you had a power with a target line "2 enemies in burst" (on a close burst power) then obviously the "select a target" step has to be iterated since it is ONE attack. But in the case of multiple attack powers this simply isn't the case, each attack stands alone as the rules clearly state.
Admittedly its possible to come up with a scenario where things could be unclear since powers are intended to be able to be designed to do pretty close to anything the designer wishes, but that's what things like secondary attacks, effect lines, miss lines, and special sections are intended to cover.