>My trick was to write the first word on each line, then the second, and so forth.

You basically used Taylor-esque division of labor with yourself. Doing the same task a lot usually you become faster and more efficient at it. If you only have yourself it still might be faster if the task is repetitive enough and your context switching time is less than the time you save by writing one word so many times.

I learned to write on a blackboard with both arms at the same time (an ability which has had me labeled a freak!). In this case, it worked great so long as nobody saw you do it, and, the phrase wasn't two long.

I knew simultaneous ambidextrousness wasn't that unusual.

Still yet to tackle that one (after I learn ambidextrousness).

my high school math teacher could - write perfectly on the board while looking at the class - write a complete sentence starting from both ends meeting perfectly in the middle

More things to aspire to with tentative hopefulness! :D

(Said as someone whose hand-eye coordination could do with some serious tuning)