So here is a video of a run up after I had redesigned the interconnecting tube. The original design used a 3/4 inch diameter tube to connect the two nacelles. I was not able to get past 900 RPM because the nacelles would start oscillating about the interconnecting tube. So I increased the diameter of the tube to 1.5 inches. This provided enough stiffness that I was able to move through the instability without destructive resonance. You’ll see in the video that I am first increasing RPM to just see how bad the vibration is and then I do a complete run up getting the rotor speed to what I anticipated was the operational RPM. Once I got past the vibration, everything smoothed out and was fine. So take your initial runup slow. If you see a vibration or head oscillating then take a moment and think whether you want to proceed. I was seeing 1 to 1.5 inch movement at the head in my original design. The difference between our designs is that for you each head is sitting on the ground so that will help things but once you get in the air, everything changes. So good ground runs don’t necessarily mean that you are safe.
One other note on your design is the location of the controller and the stiffness of the mounting location. I could see this as a potential issue depending on how the vibrations setup in the structure. And for this I’m not talking about the issue above, I’m just talking about natural structural vibrations that occur in any helicopter frame. So your initial run ups that will be a good time to see how the vibrations look in the flight controller. Again though, everything changes once you are in the air.
https://www.youtube.com/watch?v=ueEP4UFbkVw&feature=youtu.be