Marty: "The key is to get that first etalon (primary) on band with highest uniformity first. Then add the 2nd etalon and tune it. If you try to just tune both at the same time back and forth, it will be futile."
That's valuable advice! I have a double stacked LS80THa, and my process to date has been to burp both to atmospheric pressure, and then screw in the first PT knob until limb features were most visible. I would then screw in the second PT knob, hoping for an improved image. I think that I usually find it, but the interaction is uncertain and a bit confusing. I was impressed the other day, though, when I didn't burp, and just looked in after about four days. Prominances were just as vivid as the day that the PTs were adjusted. Those o-rings seem to really hold pressure well!
Per your (and Bob's, in the on band linked post) process, I would not have the second etalon in the light path while adjusting the first, and only install it after the first was on band. To do that, I will need a dummy mechanical etalon to let me attach the focuser and eyepiece with just the first etalon in place. It would be easy to measure its thickness, and Precise Parts is there to make one. I'll give it a try.
All the best,