All you have to do is record - separately - the video and audio, move them into iMovie and sync them.
Now! … having shared that piece of wisdom! … that is exactly what I have done with two files (video and audio) EXCEPT for the syncing part.
My proficiency with iMovie has been pretty darn good in the past but I've not used it for quite a number of years,
I now need someone to give me a few tips to refresh my memory.
(Online Help never seems to have the exact piece of information needed.)
Kindly contact me if you can help, please …
dickfaulks@icloud.com
Thanks!