Twin Strike is the king of at-wills, but on other hand, nerfing it heavily will mean that Rangers will drop below other strikers. One could kinda see it as ranger class feature even.
All problems with Twin Strike combos have one thing in common: Versatile Master. Feat that is as strong, or stronger than most Paragon path features. Update Versatile Master to require Paragon multi-classing as prerequisite and Twin Strike as dilettante will not be an issue anymore.
Edit: fixed typoes
It is a delicate dance between makingt he class features of the Half Elf too good or sub-par. It's the disadvantage of a feature that automatically gets stronger as the number of options increase. I actually see it as a good sign that dual strike (a PHB power) is still a leading contender . . .