My guild has been steadily improving our heroic Twin Valkyr strategy, and I think it's good enough to share what we found out.
We tank both valkyr next to the white portal closest to the door, all dps and healers take the dark essence and the tank for Lightbane takes the dark essence. The tank for darkbane and three runners take the light essence.
All the raid stacks into one point, except for the runners and tanks. The tanks stand opposite the raid, with the valkyr positioned in such a way that melee splash and cleaves hits both valkyr.
The interference runners (hunters) form a three-quarter circle around the raid, with the opening where the tanks stand. The Interference runners have as main task to pick up all white orbs headed for the raid in their quadrant. Sometimes they'll have to run through black orbs to do so, so they equip some shadow resistance gear. Dps is by far a secondary priority for them.
Should one of them fall, the other interference runners need to reposition and work harder - orbs getting through means a wipe. I've seen two hunters successfully do the task, but starting with three works best for us.
When we have a lack of hunters, we ask a shadow priest to assist.
The orbs in the free quadrant are to be picked up by the tanks - they don't take enough damage during the fight for this to be an issue.
At our current dps, the Valkyr get to do five abilities, and we worked out tactics to be able to survive these five abilities, no matter what the fifth is.
For the first four abilities, here are our tactics.
All dps is already focused on lightbane - the shield should go down easily.
We pop Divine Sacrifice and (fire resistance-)aura mastery, every dpser able to do so uses survival cooldowns and every healer uses their most potent heals.
If the dps uses their survival cooldown properly, most classes can actually live through the surge with minimal healing.
We recently discovered that the damage does not lose it's type when channeled through divine sacrifice, so if you use a light-essenced paladin tank, he can use a Divine Sacrifice without using bubble, and not take any damage. (Getting around 180 charges of powering up instead.)
It's important that the healers know who to focus on - some classes have a rather annoying lack of survival cooldowns, and these will need healing.
All the dps switches to Darkbane, except for rogues who use blade flurry. Every cooldown, including bloodlust is saved for this shield. We have four or five high-hp melee dps swap to light to break the shield. These dps will need to make their way to the black portal and back to the raid without dying or using survival cooldowns, hence the choice for high hp dps.
The dps switching to light focus should make sure to step forward so they stand between the tanks and the valkyr so they don't get instantly killed by the black orbs floating into the raid group. This means that they will be attacking Darkbane from the front, but it shouldn't matter (no parries while casting.)
The light-essenced tank will use a survival cooldown here, as should the interference runners. (A well-timed deterrence should be able to deflect all but one of the tics.)
You could run the risk of having a paladin do a divine sacrifice (without bubble - they need their forbearance for the light surge), but he should be aware that there is a tank taking good amounts of damage, so he should take some extra healing.
The easiest fifth ability is a light shield. Nothing changes, everybody just keeps on dpsing and they break the shield.
If the fifth ability is a dark shield, our healers take a step back and all dps switches to light. The healers will form a shield, picking up the dark orbs while all dps switches to darkbane for the remainder of the fight. It's a bit tricky to get the hang of, but works well once you get it right.
If the fifth ability is a dark surge, we simply do our best to keep the interference runners alive. The light-essenced tank should be able to survive with some decent healing. It helps a lot if you have additional cooldowns here. (A ret paladin using divine sacrifice without bubble is a decent choice here - while they risk death, they likely take enough damage to ensure the others survive.)
Should we lose interference runners, we have several dps able to swap to white and take their place.
The hardest ability to deal with is a light surge. This will require almost all dps and healers to swap to light, with a few stepping back to form a dark orb shield and getting severe healing. Additional survival cooldowns are extremely helpfull here, but most of them should be on cooldown.
Any questions and opinions are more then welcome.