I imagine if you have a pure heavy attack build you'd get less out of an axe since there'd be fewer attacks to proc the bleed, but it's still some good damage.
One axe on my light attack weaving Stamblade is giving me around 850 DPS bleed damage on a single target. The damage one dagger could add depends on how many active crit damage buffs I can get, and it can't possibly match that number even with warhorn and rearming trap since I'm only holding the dagger half the time.
Other weird facts:
If you use an AOE skill like power extraction while holding axes you can bleed a lot of things, but steel tornado won't proc the bleed I hear.
Two daggers would buff the ground AOEs from the bow bar as well as steel tornado.
Right now, all other things considered, I think BiS is axe and dagger. Granted there's a lot that goes into this and honestly unless you're doing everything else perfectly you're unlikely to notice much of a diff. Having one axe is usually worth it for the bleed though. Your other weapon is up to you. But the increased crit from a dagger is good. Let's just say you cant go wrong with a dagger and an axe so why not. Run both sharp and you can't go wrong. On my stam DK I'm running spriggans so I run the axe sharp and dagger Infused with poison enchant for higher single target damage.
In my opinion axe + dagger is better for constant DPS in trials, dagger + dagger when you want higher and more reliable burst in short fights like vMA and vDSA. The only exception are builds that can't reliably proc the axe bleed, but I think there aren't any of those left in the current meta.
If i remember correctly, bleed ignores resistances, so if you are wearing TFS + kragh etc, it will be better to go dual daggers,
With next patch comming up, the shadow mundus is looking really promising, so the extra crit you get from dual dagger will be better then axe if you go with the shadow mundus.
"BiS" would be to use Axe+Dagger for single target DPS. You will put out more AoE DPS by using Dagger+Dagger.
Either way, the difference is DPS is minimal.
From a viewpoint of putting out maximum DPS, I recommend Axe+Dagger since it can slightly boost single target over a Dagger+Dagger setup. The AoE DPS gain of using Dagger+Dagger is almost nonexistent.