Main dagger and offhand dagger
Is there a difference? Yes there is. The main hand's dagger's DPS is what is counted towards your overall DPS score. On the other hand your offhand dagger is only as good as it's stats, the DPS doesn't matter.
Main Hand Dagger
You main hand dagger should have as big DPS as you can get, the other stats are secondary at best.Your main hand dagger's DPS is what is affected by +% Weapon Damage (Lotus Blades feat). It also makes your face stab damage whole lot better.
You offhand dagger should have stats needed for your build, raw DPS of the dagger doesn't mater at all. Two good things to have on your offhander is +combat rating and +constitution.
Confused? Lets do some math on PvP T1 and T3 dagger.
Which one should you have as a main hand dagger? This is no brainer, Highlord's Dirk has superior DPS. Now the tricky part: should you be using 2 x Highlord dirks or perhaps Main hand the Highlord and offhand the Skrimisher dirk? Lets do some MATH!
64 Dex = 192 Combat Rating
250 Combat Rating + 192 Combat Rating = 442 Combat Rating
Added stats from dex:
+32 Evade Rating
+3.2 Natural Stamina Regen
+9.6 OOC Stamina Regen
485 Combat Rating
+3 OOC Health Regen
+1 Natural Stamina Regen
+3 OOC Stamina Regen
+26 Stamina Tap Rating
So as you can see Skimisher's Dirk is superior when it comes to both useful stats and damage. There will be some loss on Critical Rating and Critical Damage Rating, but those differences are not big enough (+8 critical rating is super marginal) to sacrifice your overall damage output. This simple example illustrates how your should think about what are you REALLY getting out of your daggers.