Salectric has spent some time in pursuit of this so hopefully he will post some thoughts on this subject. He and I have traded emails in the past on this subject. It looks like I've lost those emails. I'll check the other laptop but in case I can't resurrect that conversation I thought I'd start a thread here to capture thoughts on this subject.
I do recall that Salectric had already found that there are differences in Gavia playback when the aluminum stub is mounted or not mounted in the second arm position. He also had found that there was a price to pay when a second arm is mounted. At the time I was a little slow on picking these differences up myself when I attempted to verify what Salectric was reporting. In the end, this was because I was really behind him in finding the best material to mount the Gavia on. I've since found a combination that works very well for me so it is much easier to hear the changes involved with mounting that second arm.
My own experiments have involved fabricating a second arm board from wood. The first attempt was a quick mock-up with some Baltic Birch plywood. The result was not very impressive and left the sound smeared. I've since fabricated a second arm board out of cocobollo that is much better sounding than the plywood. No real surprise here, but it is impressive how much difference the change in wood makes. Yeah, like that would be news to Chris Brady.
I'm still formulating new impressions of dual arm mounting since elevating my Gavia on Stillpoints. I see this whole dual arm mounting as an issue in vibration control. Since that is what the Stillpoints excel at, I'm expecting things to change. Whether or not the change is better, or good enough, remains to be seen.
Maybe if I can get this to work well Thom will make me an arm board from aluminum.