What obvious fubar am I am missing?
I am thinking that the gotcha will be timing even if you rotated the part 180 degrees.
I will admit that i never did any thread milling.
For the lathe, the G32 threading cycle index triggers the movement of the Z and feed rate is sampled and adjusted so that
the entry point would be exactly the same based on rpm of the rotating stock.......in the case of a 2 starts, Z is adjusted to offset the thread start by 180 degrees,but you still have the monitoring of movement from the new start point. For the mill you have no way of having the mills z travel monitored to make sure the 2nd thread pass enters the piece 180 degrees from the first pass.
Any small difference in total Z travel timing (distance relative to acceleration and feedrate and also start point) will affect the entry point of the cutter and it doesn't take much to throw it off by some amount of degrees. So the distance between pitches will not be correct. I am guessing that one could do some testing and see the how it goes but the outcome will be hit or miss.
FWIW
RICH