It doesn't matter more for the off-hand than for the main-hand. You will still use the highest median damage weapon in main hand. Median damage is (Max+Min)/2, if you use median damage you can ignore speed because quicker weapons will automatically have a lower median damage.
And since you are dual wielding you will have a penality to hit (and damage) to your off hand. So your priority should be your main hand because of special abilities (opposed to auto attacks). As for the off-hand you will want slow weapons (and high median damage) because unlike rogue who will want to apply quickly poisons you will want to do more damage with each hit.
Higher median damage is always better for a death knight.
Blood Deathknights should be spending their runes to use Heartstrike (which they get as their Blood Specialization) and Deathstrike (which is the focus of their Mastery specialization).
Talents
Here is an example talent spec: http://www.wowhead.com/talent#jcbG0srMusd
The core talents you want to be sure to pick up are Blade Barrier (reduces damage taken), Toughness (increases your armor), Bone Shield (tanking cooldown), Sanguine Fortitude (improves Icebound fortitude, a tanking cooldown), Rune Tap and Vampiric Blood (both tanking cooldowns) Improved Blood Presence (Crit immunity), and Dancing Rune Weapon (survivability AND threat cooldown).
After that, you have a choice between dipping into either Frost or Unholy. Unholy is almost always going to be strictly better, as you do not want to dual wield to tank (dual wielding has an inherently higher miss chance than 2-handed weapons, so your threat is going to be much worse) nor are you using obliterate (as you should be using death strike). Unholy gives you your choice of reduced taunt cooldown (Deathgrip), free +hit% for your spells (so deathcoil won't have a chance to miss) and morbidity (boosts the damage and thus, threat, of your runic power dump: deathcoil). Taking frost as far as Lichborne, however, means you can activate Lichborne, and then cast deathcoil on yourself to get a free heal if you're low -- a neat trick, and handy if you're low on health after a big boss burst.
Glyphs
Prime Glyphs:I would go with glyphs of Death Strike, Heart Strike, and Deathcoil for your prime glyphs, though depending on playstyle you may prefer to swap the latter for Runestrike (though I'll always take +damage over +crit chance, since the former is more consistent) or Death and Decay if you want additional AoE threat.
Major Glyphs: Dancing Rune Weapon is a no brainer, but beyond that your choices highly depend on your playstyle.
Minor Glyphs: I'd Glyph Blood Tap and Death Grip, but these are minor glyphs, and won't have much effect at all in regards to your tanking.
For further discussion, I'd point you toward the Elitist Jerks' Death Knight Forums, specifically the tanking thread, which can be found here.
Best Answer
Yes, all classes can learn dual talent specialization at level 30. However it first must be purchased from your trainer for 30g.
Simply return to Acherus, the Ebon Hold and talk to any of the trainers that would normally teach you new abilities. One of the speech options with the trainer should be "Learn About Dual Talent Specialization". Once you have done this, there should now be an extra tab on the right hand side of your talent trees to switch between the two talent specs.
You may need to have completed the Death Knight quest line to unlock this option. E.g. be level 58.
Image source: Joystiq.com