I tried 2TCing again today. I picked a basketball-sized, limbless tree. My last climb I got my feet to 20' in 20 moves, in ≈9:08 running time (paused my stopwatch a few times because I had to stop and rest).
I can tell you right now, the way I one-stick with moving the tether up every step to avoid slack, I cannot one-stick to 20' faster than that. So I feel like I'm really close to going all-in on 2TC. I think today showed me although the individual moves feel like a lot of exertion, I felt way more coordinated than on previous attempts (choosing a smaller tree probably helped) and I'm confident I can get faster. Yeah, I was a sweaty mess in 75° sunshine but I would've been one-sticking too.
I think the only hindrance in my mind is wet trees. I imagine 2TCing a slick tree trunk would be miserable.