Your problem come with thinking that an Epic Magic Weapon could possibly overcome Damage Reduction the reguires an Epic Magic weapon.
See.
Doesn't that make sense?
An Epic Weapon doesn't overcome Epic Damge Reduction. In fact, non-Epic wepaon are pretty good at overcoming Damage Reduction that can only be overcome with Epic Weapons.
In summary:
An Epic Weapon is not an Epic Weapon.
Follow.
Me neither.
This seems to be a case of One editor having no Clue what another was thinking, and now we have to live with it.
DR X/Epic, is in fact DR/+6. When 3.5 got rid of all the +1/+2/+3 Etc for Cold Iron/Good/Etc, they decided to change the name of DR/+6 to DR/Epic.
The problem was that the Epic Level Handbook had already spelled out what an Epic Weapon is (+6 or Higher OR total bonuses over +10).
Rather than merge definitions of Epic (or change the name of one of them), they decided to keep both of them.
So I can say "Sorry, this Epic Weapon won't overcome his DR 10/Epic because you don't have an Epic Weapon, but his Non-Epic Bane Weapon easily overcomes the DR that requires Epic Weapons to overcome" and be 100% Correct.
Sort of like saying "I'm sorry, but those Rails just won't fly, but those Rails beneath the Train are about to get crushed" and be 100% Correct (as I'm talking about Rails, the bird & Rails, the things Trains run on).
D&D just came up with 2 different definitions for Epic as it applies to Weapons & Combat.
There are 2 very simple, VERY, VERY, VERY, VERY SIMPLE ways of resolving this that seems to complelty baffle the R&D guys at WotC.
1. Change DR/Epic to DR/+6. That fixes it right there. Switch 4 Letters to a Symbol & a Number. If its Epic & under +6 No Confusion. If its not Epic, but +6, No Confusion. No confusion about Weapons of Legacy, Intelligent Items, or Artifacts.
If they are not +6, not gonna work.
This is the fastest, simplest & most in tune with the DR/Epic description in the MM.
2. Define Epic. One Definition. Stick with it. Go with the one from the Craft Epic Items Feats or the MM DR Definition. (as the DR has no guidelines for creation, or Modifiers over +10; the Epic Feats one is the way to go).
An Epic Weapon is one created by the Craft Epic Arms & Armor Feat. Anything Else just won't cut it.
Option 2 is what I go with. It requires some adjudication on your part. (I let the +5 Bane Weapons Sneak in as a Loophole, I also allow some High Level Weapons of Legacy & most Greater Artifacts act 'as if Epic' for overcoming DR/Epic).
All in all, as long as you realize that Epic doesn't mean Epic, you'll be OK.