This was working on PTR, and I just tried it on live (these screenshots are from a few minutes ago). If you cast mark of the wild first, and then blessing of forgotten kings from the drums, they will stack, and provide the increase. I have noticed that the forgotten kings dissapears if you zone though. It will not work stacking kings and forgotten kings, and Mark of the wild has to be cast first.
Attachment 4150
Attachment 4152
Attachment 4151