The key to killing this boss is the beam between two bosses. It appears when bosses are close to each other and switches off when you separate them. The safe approach is to have one tank stay in middle of room while other stays near walls. During course of fight tanks will connect and disconnect bosses to create and shut off the beam.
That beam is used to break shielf of Dwemer constructs that spawn on positions around room. Beam will melt anybody who gets into it so bosses should remain connected only for as long as nessesary. Fight is full of all sort of stray damage, AOE and massive dots (10k+ per tick). Tanks generally run own purge. Tank who stays on edge of room will be guiding his boss to Dwemers and tank in middle will be making sure to connect bosses and disconnect them ASAP. This liquid lightning dropped by boss will melt even a Tank, need to roll out of it ASAP. During fight tank might get knocked down by boss which will start feeding on him. Assistance (interrupt) required from other player. One thing that struck me during this fight is that despite all the damage and difficulty, the fight iteself is very player friendly. As long as you follow mechanics (kill Dwemers or interrupt them) you can take your time killing bosses, its also easy to recover. There are no hard DPS checks or enrage on bosses, notice that even when tank dies group is being able to recover having as low as 4-5 people alive.
The reason bosses are swapped is that we want them to die at same time. DPS always focus boss on the outside. When one of bosses dies too soon there is no beam to break spheres shields and also spheres become enraged. At 85% boss jumps away to break buff statue. Normally group wants to throttle DPS to first break Gargolyes. At this point group must focus not only on killing boss with more HP but also to limit any stray damage on boss with low HP. When one of bosses dies the only thing left is burning down last boss and kepping spheres interrupted at all times so they dont spread poison injections.
As found on Youtube