Can thieves ever hit with more than one thrown dagger?
Thieves and Throw...
Yeah... but you'll only ever see one label of damage depending on 'how many' hit. If you throw and hit with none, obviously it's a miss. If you throw and hit with one, the damage will be pretty weak. Throw and hit with some, it gets nice, and throw and hit with a mass of them is where the big damage is. This is a rough idea of how it works, I could be slightly off, but that's the gist. You won't get more more than two 'words' of damage though.
but I've not seen thieves hit with more than one EVER...am I just blind?
Maybe it's only ninjas that can hit with more than one... but I've played ninjas and thieves more than two of each. When you edge things as a thief... you make more than one knife... so I imagine you could hit with more than one. Maybe someone more experienced than me will spot this thread. 
Thieves and ninjas throw has been greatly toned down. At the moment I believe it is not possible for them to hit with a "mass". All the other "lower" damages are possible though. I have seen OBLITS with their throws. In the past though, it was possible to get ===DISI===. Only Bards' throw has not been changed and it is the strongest throw from those three classes.
Thieves only ever hit with one dagger, but their daggers tend to do the most damage per dagger than any other class.
Ninjas can hit with more than one, their individual shuriken is weaker than a thieves individual dagger, but the ability to hit with more than one makes their actual throw able to do more damage.
Bards have maybe the strongest overall throw, because they can hit with the most amount of cards, but their cards tend to be weaker on average than both thieves and ninjas individually.
I think this is right ... but I could be way off.
Yeah you are right Nekky. I've had a bard hit with one on a throw and do like an injure where my thief hitting with one would do at least LACERATES. On the other hand as a bard I've hit with a mass and done something like MASSACRES or whatever is after that in the damage scale.
It's possible to do more than a mass with cards I believe, if you're really lucky. A 'cloud' of cards.
Thief throw damage can be increased.
Dey
Yeah I've seen 'cloud' before. It seemed like it was pretty close to the same damage as 'mass' as I remember, though I could be totally wrong (it was like 5 months ago maybe.)
I believe is only bards that can reach the "Cloud" level. 
EDIT: I think it would be pretty cool to see the individual damage from each throw. Like magic missile. Hehe.
Also look to see if you have autoaim mob on. If you do I believe it will give you increase chance to hit, less damage.
If I recall correctly, thief throw damage was lowered because of the absurd damage of dual backstab, traps and autothrow combined. Although, now it has a chance to wound the target instead, which is pretty neat.
Because thieves one-rounding you is great.