Of the entire course of an adventuring career it could be argued that mathematically they compete in the same space, but that is irrelevant because what actually matters is whether it gives players a valid choice to make about which to use in different circumstances. And it does.
Well, no, not really. The aggregate differences between the two certainly add up over time, but even on a single die roll, Twin Strike is significantly better.
Rolling twice is, statistically, about the equivalent to getting a +4 bonus on an attack roll (there's some more wonky math in there, but that will do for a simplification). A single hit with one Twin Strike attack will do the exact same amount of damage as a hit with Careful Attack. Effectively, you're choosing between a +2 to hit and a +4 to hit
and a decent chance of doubling your damage output.
Consider: You attack a monster. You'll hit the monster on a roll of 11 or better if you make an unmodified attack, and you're using a bastard sword.
- Careful Attack drops your hit requirement to 9--you now have a 60% chance to inflict 1d10 damage.
- Twin Strike lets you roll twice. That means that, with a hit requirement of 11, you have a 75% chance to inflict 1d10 points of damage, and a 25% chance to inflict 2d10 damage. When you start adding feat and enhancement bonuses, that double damage gets even better.
As written, there is literally
no circumstance in which it is, from a mathematical perspective, better to use Careful Attack than Twin Strike.
(if you are concerned about powers getting in each others face in the same space then 4e must be a bit of a nightmare, with all kinds of powers doing almost exactly the same thing but with a tiny differentiation in area or duration or some such!)
Cheers
There are very few powers in 4E that
directly compete the way Twin Strike and Careful Attack compete. Either similar powers are on different class lists, they're different levels, or their additional effects are different enough that using them becomes a tactical choice.
