Formula for single hit damage

[Concillian]

Thank you for posting this.
Other questions arise. 

1. How much more damage does a faster dagger have to do before it becomes more desirable than  Broad Blade Knife in the main hand?

It depends on how much AP you have. As was demonstrated, speed is a very large factor in terms of Ambush and Sinister strike damage.

This is why dagger rogues farm up Toxic Revenger

Also,
Tigerbane (farily common drop)
and Silent Hunter (obtained from Horde quest in Arathi)
Are popular, easier to get, alternatives to the blues.

Also Fiendish Skiv (Horde quest in Duskwallow) but that is higher level, I think around 45 or so.

2. Under what circumstances is it more desirable for a rogue to use 2 fast daggers with high DPS?
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Umm, none?
I guess if you went with slice and dice + poisons it might work out, but overall mainhand = slow and offhand = fast. Physical offhand damage is cut in half, but poison procs are full damage, so a substantial amount of extra damage can come from the off-hand in terms of poison damage. Posion damage percentage difference will be simply the ratio of the weapon speeds. so going to a 1.3 speed weapon from a 1.5 speed weapon, poison damage picks up by 1.5 / 1.3 (~15%)