Assume the normal twins paradox. Let's call them O and O' where O is the stay at home twin.
The non-inertial twin will accelerate at "a" for time Ta through the 4 acceleration phases.
Also assume there is another observer O2.
Finally, when O and O' are at the same place and at the instant O' accelerates, a light pulse is emitted. Further, at that same instant the observer O2 will be at the same v that the O' twin will accelerate to and that observer is also co-located at the light pulse with O and O'.
What time will elapse on the clock of the stay at home twin while O' elapses Ta?
How will the center of the light sphere travel in the view of the accelerating twin from the frame of O to the frame of O2?