Property: When using a power, if you hit one creature with both your main weapon and your off-hand weapon, deal an extra 2d6 damage to that creature.
If I'm using twin-strike, when is the damage applied, and is it ever maximized due to crit?
I think there are two possible readings of this - the one that I would support, as it does not introduce any weird contradictions, and does not feel grossly overpowered:
When is the extra damage applied? Once you have resolved both the main hand weapon attack and the offhand weapon attack.
Is the damage ever maximized due to a crit? No.
And the one that I would not, based on reading an awful lot into the word extra:
When is the extra damage applied? As bonus damage dice to each of the attacks.
Is the damage ever maximized due to a crit? Yes, probably, as they appear to be bonus damage dice.
But then for powers that allow for main and off hand attacks, but have one triggering on a hit from the other, do you apply retroactive damage?