It used to be that ppl chose 1h dagger for pvp, since it had critical dmg SA.
But after looking at r grade weapon lists, they have similar SA's, and none of them come with critical dmg. They also seem to have the same extra bonus when enchanted to +4.
Odd how no actual dagger players have commented on this thread yet.
What you could do is get to 86, make a new 1-40 toon, get both types of requiem daggers and test if you find you like one better than the other. Before you try getting apo daggers.
So far everything i tested on my daguerr its better always duals daguers in pve adn in pvp: single daguer seem to have more damage on attacks but as on duals give better dps and better damage in general. Obviusly you spend more ss but you do more damage you choice :D
Dual daggers if you can afford it. To be honest the best daggers are top grade s80 ones. They have the pvp option which adds a considerable bit of damage in pvp. Plus they scale with your lvl. No self respecting dagger would carry anything other than dual s80 in pvp.
I did the single dagger thing for a while, but kept falling back on my +4 dual apocs. They do consistently more damage, plus have a bit of panic inducing benefit when someone is getting hit with duals at 1500 atk speed. The single dagger supposedly half-kills more often, but I haven't noticed much of a difference. The best thing you can do is go +4, and get 300 element.
Dual Daggers with Damage Up SA are the best in PvE. They are good in PvP too, but a single dagger with Haste SA might be better in Oly, depending on your base stat/buffs. The HK difference between duals and singles isn't noticeable. OE-ing both duals and singles might be costly, so I'd just stay with duals for cost-efficiency.