When each of the twins is accelerating, it's harder to determine the perspective of each twin, but it is possible. When I joined this forum, I spent a lot of time doing searches on the twin paradox, and I remember that several old posts did talk about how to get the perspective of each twin when they are both accelerating. So far, I haven't been able to find any of those posts again. Maybe some other forum members remember them, and maybe they will have better luck finding them than I've had. I don't think Brian Greene talks about that kind of situation in his book or in the NOVA series, though.Can you please go through the steps to show us how you would get the answer to the OP's first question in his section 2b?