as I was readig the changes for bypassing DR I noticed you do not mention creature that have multiable needs like oh a Lich which in 3.5 has DR Magic and Blunt or a Demon that has Magic and cold Iron
how will that work does that mean that a +3 Cold Iron sword still get the +2 bonus to damage because a +3 weapon by passes cold iron or not because the creature needed both in the 1st place?
Quote from page 153.
"Such weapons deal +2 points
of damage on a successful hit against a creature’s whose
DR they overcome. This bonus does not apply against
creature’s with DR/magic."
So in both your examples the monsters have "magic" as one of their DR types, and hence, PC's would get not get the +2 damage bonus against these monsters, ever.
I think that while you should not get a bonus for overcoming DR/magic, the magic component of composite DR should be ignored for the double overcome bonus.
Example: A critter has DR/magic and cold Iron. I'd say that a +2 cold iron weapon (which overcomes DR/cold iron twice over) would get the +2 damage bonus, since the magic part is ignored for this.