When you kill a redfang furbolg you gain advantage with frostpaw
and vice versa.
So when you are doing the quests that require you to enter into Grizzlemaw you can kill one and then go in and reduce the aggro you get from furbolgs.
[Common sense]
BUT! If you kill 2 furbolgs at the same time, of the 2 different tribes you get both buffs. So you can go into grizzlemaw for 5 minutes without getting attacked.
Have fun.
You gotta kill them at the exact time! With something like AE, Holy Nova or something. That kills them on the same tick.
This is in Grizzly hills btw.