Hmm. So essentially Double Swing hits faster, but not as hard as frenzy. I wonder what the damage per second difference is.
I'm copying and pasting this information from a post I made elsewhere, so forgive me if it's a touch disjointed.
Here's a very, very rudimentary comparison involving Berserker Axes with 20% IAS. I'm also operating under the assumption that it takes 3 hits to fully charge a Frenzy (personally, I believe it takes more than that, probably 5 hits, but by using the 3-hit value I can give a very conservative comparison biased in Frenzy's favor). This means the first hit will occur with 0% EIAS, the second will occur with 14% EIAS, the third will occur with 28% EIAS, and the fourth (and all subsequent) will occur with 43% EIAS.
Zerker with 20% IAS and Double Swing = 6.5 fpa
Zerker with 20% IAS and 0% EIAS = 12.5 fpa
Zerker with 20% IAS and 14% EIAS = 10.5 fpa
Zerker with 20% IAS and 28% EIAS = 9 fpa
Zerker with 20% IAS and 43% EIAS = 8 fpa
Time for 5 hits with Frenzy = 48 frames. During that time you can launch SEVEN double swings (and be 40% of the way through the 8th). For every subsequent hit, Double Swing will fire 23% faster than Frenzy, (which should correspond to multiplying DS's final damage by 23% compared to Frenzy's).
Now, same comparison, except using Champion Swords and 40% IAS
Double Swing = 5.5 fpa
First Hit Frenzy = 8 fpa
Second Hit Frenzy = 7 fpa
Third Hit Frenzy = 6.5 fpa
Fourth+ Hit Frenzy = 6 fpa
Time for 5 hits with Frenzy = 33.5 frames. During that time you can launch six double swings and be 10% of the way through the seventh. For every subsequent hit, Double Swing will fire 9% faster than Frenzy (which should correspond to multiplying DS's final damage by 9% compared to Frenzy's).
Taking these figures into account in the 40-point Frenzy/DS comparison, Double Swing will actually leave Frenzy's damage in the dust. And remember, too, these numbers were FRENZY FRIENDLY, operating under the assumption that Frenzy is fully charged in 3 hits (which I'm almost certain isn't true). In reality, Double Swing's damage is a lot closer to Frenzy's with 60 spent points than most people would believe (especially if you factor in the greater chance to hit, and even more especially if you are having trouble keeping Frenzy charged- say, you're playing in a group and don't want to race ahead), and the only reason Frenzy feels like it's killing so much faster is because you spend a lower percentage of your time running between monster packs.
Basically, the d/s difference is going to be negligible. It might favor Frenzy slightly, it might favor DS slightly, it's going to depend on your setup... but either way, the difference isn't going to be very big.