The only vibration they act on is 4 per rev. NO other frequency is helped by the bifilar.
There's something I didn't know..
The orientation of the arm over the space between the hub arms is specifically designed to allow ease of maintenance and to prevent interference with blade flapping. There is no issue with the exact orientation, the weights all act to absorb the in-plane vibrations.
Thanks again, common sense I suppose, however, I appreciate the post.