Very simple exploit to skip every other beast.
When beast of nightmare goes out, don't kill the add right away, get it really low health so you can kill it in 1-2 globals, when the next beast is being summoned (other tank gets a countdown for when it spawns) have the original tank kill their add, this will stop the other tanks beast from spawning.
1, Tank A gets beast and brings it to low health
2. When Tank B gets the debuff that says a beast will spawn, have tank A kill his beast
3. Beast of nightmare won't spawn
The debuff goes on the tank like 4 seconds or so before the beast actually spawns and can be targeted, that's the gap of which you have to kill it. If you kill it to early (before debuff) or to late (after beast is attackable) it won't work.