"Always move clockwise" tactic works for me. Just face the dark boss away to avoid random negates. Explosion range is actually not very huge and usually I'm able to heal and buff people without any issues, even if Im in the opposite group.
Make sure that everyone interrupts white boss and his adds, it might sound super obvious, but its really important here, because allowing him to beam a tank or healer can easily cause a wipe.
If you're targeted by dark boss' adds (mages), keep in mind that they do a ton of damage to the opposite group so if they aim to the group, move a bit to the side so their spells wont wreck everyone.
Also, efficient purge makes things much easier since it removes fire dot.

Originals is the easiest strategy to remember. So in the phase where no-one has a colour, everyone goes back to his or her original boss. The original boss is the one you attack right at the beginning of the fight, before you get any colour and tanks exchange the bosses.

Repeat that until the bosses are dead. In the case that you have to switch sides, everyone moves in clockwise direction.

One chainer on each side, but the tank that has the white boss can usually taunt the white adds as they're melee and come automatically, so deep breath to interrupt the adds is usually enough.

Within our speedruns the healers/chainers call out who got to switch, no one else does switch otherwise. In earlier stages of our group, we've used the original tanks strategy. (If you've been colorswitched, do run back towards the opposite direction.) Less runs > more dps on boss.

I'm also familiar with the original bossess, which is basically the opposite of original tanks. (If you've been colorswitched, do NOT run back towards the opposite direction.) In less experienced runs (outside my main guilds), we're usually keeping the boss near the doors. In speedruns and such, bossess are placed closer towards each other, which requires more attention, but also gains a smaller running distance, and thus more dps.