unholy in twin valkyr fight (toc25 n and h)

i need you guys to give me tips on how to get the most damage in this fight, pressence, when to use gargoyle, anything, thanks!

Use Blood Presence and spread your diseases (Pestilence) to both Val'kyrs if possible.

You want to use your gargoyle with bloodlust/heroism. Although, if the fight lasts seriously longer then 3 minutes it's more useful to use the gargoyle twice, hence use it at the beginning and when it comes off cooldown. Having a potion of speed ready to use with your gargoyle is a good idea.

To this point, it's nothing different to any other DPS encounter.

However, to deal the most damage possible you have to utilize your color debuff.

The obvious part is to attack the opposite color target of your debuff (e.g. dark debuff attack Lightbane, light debuff attacks Darkbane). Further, you want to collect as much of the same colored - as your debuff - orbs as possible. Each time you are hit by a same colored orb you gain stacks. At 100 stacks you gain the Empowered Light/Darkness buff. That's a 100% damage increase for 20sec.

Dont forget to Death and Decay...

While it's correct that generally for 2 targets, DnD is a gain over 1x Scourge Strike + 1x Blood Strike + 0.375x Death Coils, it doesn't apply on Twin Valks because of the way Light and Dark essences work. Your damage is drastically increased on one of the Valks but significantly decreased to the other. This favours using the runes on Scourge Strike and Blood Strike instead since those concentrate their damage into the target you deal more damage towards.

DnD should always do more DMG than SS normal hit and BS crit, on top of that since u can gain a lot of RP from AMS, and use it for extra Death Coils, 1 less GCD is also helpfull there...
A good DnD will always result in more overall dmg on 2+ targets regardless of a fight in WotLK (not counting fights like DBS where adds have 90% less aoe dmg taken buff).

Edit:
The reason why its good for low geared DKs is low crit chance that they have, so whenever ur SS and BS deals normal dmg, and ur DnD was not on CD, u couldve done more dmg on multitarget DPS...

You deal 50% more damage to one boss and 50% less to the other (I can't remember the exact damage multipliers off the top of my head but I think they're both around 50%). This means instead of getting 2x DnDs worth of damage you get only 1.33 DnDs. This should make the SS and BS a good bit better.

In that case, yes, if u are getting a 50% dmg reduction when dpsing the oposite color u might be right...
But again, DnD should almost always do more or same amount of DMG as SS and BS normal hit, even on single target, when u want to use AMS and get couple of extra Death Coils out of it, i still stand on my point that u should use DnD.

Yeah it seems to hinge on whether you get to use the extra GCD to pull off an extra Death Coil or Blood Strike. And I guess the DnD glyph (if you opt to use that) would naturally influence it too.