Strategy

Usual Raid composition:
Tank: Paladin
Melee DPS: Rogue, Death Knight, Enhancement Shaman, Feral Druid
Ranged DPS: Balance Druid, Mage (also have used a Warlock instead of Mage)
Healers: Holy Paladin (also have used a Restoration Shaman), Discipline Priest, Restoration Druid

The two twins enter the room near the front portals.
We usually use 2 "soakers" to attune themselves to Dark Essence, while the other 8 players attune to Light Essence. *Usually* we have a rogue and a feral druid soak, but honestly, if you have other raid members that have good hand-eye coordination, then anyone could could be a soaker. If your raid is really pro, you can do this with just 1.
Soaker Job: Protect the rest of the raid from incoming dark orbs. Since this is only 10 man, our soakers will be covering a larger angle of incoming orbs. Simple.

In our 10 man strategy, we prefer to single tank the twins nearest the front Dark Portal. We like to have everyone in the same general area, not necessarily stacked on top of each other, but in a loose clump. We use 3 healers, our preferred healer makeup is resto druid, holy paladin, and a disc priest. This can also be successfully healed with a shaman, druid, disc priest combination.

We Start by DPSing Eydis Darkbane (twin next to dark portal with the black wings). Dodge orbs as necessary, the soakers can't get 100% of the orbs all the time, so be prepared to take a few steps forward/backward/whichever way that will get you out of immediate danger if one or more happen to come into the area where the raid is standing. Usually we have our soakers call out on vent if they know they can't get the orbs from a certain direction and they see a lot of them headed in the direction of the raid.

We generally stay in this position throughout the fight. Usually if there is movement to the Light Portal: Front area, it is called on vent. That sometimes happens after the entire raid group has switched colors etc..
We DO switch colors for: Opposite color vortex, opposite color shield.
We DO NOT switch for Touch of light/dark.

Concentrated Essences
What I like to call "orbs" float around the room. When a player of the same Essence absorbs it, they gain Powering Up. When a player of the opposite essence absorbs it, they take damage. (See Unleashed Dark/Light)

Powering Up — Causes Empowered Darkness or Empowered Light at 100 stacks.
Empowered Darkness — Increases damage dealt to Light targets by 100% for 20 sec.
Empowered Light — Increases damage dealt to Dark targets by 100% for 20 sec.

Unleashed Dark — The Concentrated Darkness releases its energies inflicting 12188 to 12812 Dark damage to enemies within 8 yards.
Unleashed Light — The Light Ball releases its energies inflicting 12188 to 12812 Light damage to enemies within 8 yards.

Melee: Both Twins melee for about 7.5k, with Twins Spike active on the tank damage is increased to 9k. (These numbers may vary between normal and heroic and 10 vs 25)
Twin's Pact — The Twin's Pact heals for 20% (Hard mode 50%) of their total HP. 15 sec. cast. Begins cast after Shield of Lights/Darkness has been cast.
Power of the Twins — When one twin begins casting Twin's Pact, the other twin gains Power of the Twins for 15 seconds. This buff increases the twin's damage by 20% and enables her to dual-wield. While Power of the Twins is active, each successful melee strike by the twin gives her a stack of Val'kyr Quickness which increases her attack speed by 10% and lasts until Power of the Twins fades.
Twin Spike — The Twin Spike inflicts 100% weapon damage and increases damage taken by 20% for 15 sec or for 10 charges.

Eydis Darkbane
Shield of Darkness — A Shield of Darkness that protects the sister from harm absorbing 175,000 (Heroic 300,000) damage and prevents spell interruption for 15 sec.
Dark Vortex — Eydis Darkbane beings to cast a powerful vortex of Dark. Channels for 5 seconds inflicting 5850 to 6150 damage every 1 sec for 5 sec. 8 second cast
Surge of Darkness — Pulsing a Surge of Darkness Inflicting 1500 damage every 2 sec to non-Dark enemies.
**Hardmode abilitiy below****
Touch of Darkness — The Val'kyr's Dark touch has affected you, inflicting 5850 to 6150 Dark damage to players under the effect of Light Essence every 2 sec. The Touch of Darkness will not harm targets with the same attributes as it.

Fjola Lightbane
Shield of Lights — A shield of light that protects the sister from harm absorbing 175,000 (Heroic 300,000) damage and prevents spell interruption for 15 sec.
Light Vortex — Fjola Lightbane beings to cast a powerful vortex of Light. Channels for 5 seconds inflicting 5850 to 6150 damage every 1 sec for 5 sec. 8 second cast
Surge of Light — Pulsing a Surge of Light Inflicting 1500 damage every 2 sec to non-Light enemies.
**Hardmode ability below**
Touch of Light — The Val'kyr's Light touch has affected you, inflicting 5850 to 6150 Light damage to players under the effect of Dark Essence every 2 sec. The Touch of Light will not harm targets with the same attributes as it.