We are currently on twin emperors, and before trying the encounter, after reading various boards and available information, i came to the humble understanding that after a teleport, both of the emperor's aggro lists reset. Thus the person closest to him in range will be the one to start eating bolts.
Happy with this conclusions i even came up with this drawing
http://jipakazoid.googlepages.com/emp-trans.gif
in order to explain warlock-warrior pairs the transition stages.
After our night of trying though, our warrior/warlock pairs reported strange behaviour, that sometimes the warrior will be the one to first get the aggro, even if a warlock was standing closer. While this ringed weird to me I decided that perhapse it is a known issue with the encounter. Futhermore, due to the fact that Vek'lor's AOE after teleport was rather devastating, i advised the pairs to focus more on avoiding the AOE then getting the transitions perfected.
Today will be our second day of trying, hoping we will be able to progress much futher. The question i pondered over was if my assumptions were correct (first in range = first in aggro) or would spending more time on perfecting this would be a complete waste.
I can assume from Gurg's last post that there is a certain way of verifying being first on aggro. The question would be, is it what i think it is, or there are leet-ej-tricks yet to be revealed :)