Ray, when someone asks you if you're a god, you say, YES!
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I can't help it; every time I see the Twin Val'kyr in ToC, I think of Ghostbusters. The voices, the boss models, everything. I half expect the Stay-Puff Marshmallow Man to show up at the end of the encounter...
The Twins are a fun and challenging encounter that requires good dps coordination as well as a thorough understanding of fight mechanics. As is typical when fighting any "twin" mobs, they are linked in health and abilities; you must manage both of their abilities, switching targets between the light and dark twin depending on their actions. This is done by clicking on either a light or dark portal, and targetting the opposite colored twin. While it sounds confusing, in practice it's pretty straight forward. In addition to being aware of target switching, raiders must be aware of the environment, avoiding opposite colored balls of light (which can damage them) and gettting to like colored balls of light (which provides a power up to increase damage). TDC got the hang of this fight very quickly, and downed the Twins in typical TDC fashion.
Which one was the Gatekeeper and which one was the Keymaster? I don't know, all I know is to aim for the flat-top...