Question: how to connect one airpod to a different device?
Answer: turn on bluetooth on iPhone, turn off bluetooth on Mac.
Insert one airpod into your ear, keep the other airpod in case.
connect first airpod to iPhone (then try playing some music)
turn on bluetooth on Mac.
on Mac, click connect to airpods.
immediately open case and insert 2nd airpod into ear. ( ... 
VOILA!