I personally find that ranger multiclassing in a rogue is kind of worthless. Having an extra +2d6 is nothing compared to having it with the myriad chances of getting a sneak attack almost every round, which can deal significantly more damage. If your using daggers (which I recommend you should...
Two-weapon fighting in 4e is basically +1 damage to your main-hand weapon as long as you're wielding a main-hand and off-hand weapon. However, you don't suffer any attack roll penalties(fortunately) for TWFing.
For the power question, all powers are done with the main-hand weapon, or...