NOOOOOO!!!!!!!! See posts 1, 16, 27, 33, 49 in this thread to see why this doesn't work (for Careful Attack anyway... you could debate its utility for Sure Strike, since it doesn't compete with a Twin Strike-like power).
I see your point.
Let's take level 30 since it is most obvious there.
28 Dex plus +6 bow + Hunter's Quarry + Agile Hunter + Lethal Hunter + Weapon Focus + Devastating Critical + Epic Resurgence + Triumphant Attack
With Careful Attack, this is D10 + 9 + 6 + 3D8 + 1 = 35 points of damage on a non-critical hit.
With Twin Strike, this is D10 + 6 + 3D8 + 1 = 26 points of damage on the first non-critical hit and 12.5 more points of damage on a second non-critical hit.
Careful Attack = .55 * (35) + .05 * (39.5+5.5) = 21.5 average damage
Twin Strike =
.0025 * 69 (first criticals, second criticals) +
.0225 * 59 (first criticals, second hits) +
.0250 * 46.5 (first criticals, second misses) +
.0225 * 59 (first hits, second criticals) +
.2025 * 38.5 (first hits, second hits) +
.2250 * 26 (first hits, second misses) +
.0250 * 46.5 (first misses, second criticals) +
.2250 * 26 (first misses, second hits)
=24.64875
Twin Strike does more damage and has more utility (e.g. specials on more criticals, kill 2 minions in a round, etc.).
This is with a 50% chance to hit with a Basic Attack. If one were to decrease that to a 40% chance to hit (which is quite common for solos or elites):
Careful Attack = .45 * (35) + .05 * (39.5+5.5) = 18 average damage
Twin Strike =
.0025 * 69 (first criticals, second criticals) +
.0175 * 59 (first criticals, second hits) +
.0300 * 46.5 (first criticals, second misses) +
.0175 * 59 (first hits, second criticals) +
.1225 * 38.5 (first hits, second hits) +
.2100 * 26 (first hits, second misses) +
.0300 * 46.5 (first misses, second criticals) +
.2100 * 26 (first misses, second hits)
=20.66375
Twin Strike still averages more damage, but not by as much as it does against easier targets.
This means that just a small tweak could balance these out.
For example, +1 to defenses for using Careful Attack (plus mod damage). Careful Attack becomes "being careful".
The real issue here is that Twin Strike is more potent than most any other At Will power in the game system (even multi-target ones like Scorching Burst). That makes it hard to compete against.