SAVING A FILE IN 2 PLACES SIMULTANEOUSLY

Time Machine does automatic hourly backups of everything that you have and add to the iMac.

see > Back up your Mac with Time Machine - Apple Support

CarbonCopyCloner can be set to update at different times, intervals or when connected.

see > https://bombich.com/

Both can easily be updated when new files are added using the menu bar shortcut.

How about an Automator Quick Action that allows you to select an initial (one) file and then directories where you want that file copied too? If you have a drive icon selected on the Desktop, the copy also goes there. Here is the result of selecting an arbitrary PDF file on my Desktop, a PDF folder on my Desktop, and a mounted exFAT USB stick:

This is not restricted to two destinations.

The easiest might be to create a folder action that copies the files in a folder to a folder on your Mac and a folder in the cloud.

Create a workflow in Automator on Mac - Apple Support

Some of the ideas being proposed will definitely work to make a of files from one drive to another, but for your particular purpose may not meet you needs. For example, using a backup utility only works over a few hours unless you have it backup every 5 minutes (and then it can be a nuisance in terms of computer resources). If you edit your files and then shut down your computer there is no guarantee the most recent version is on the external device. You need something that will constantly be checking the file status to see if it has been modified and then make a copy of that modified file to a destination.

Conceptually this is a simple task, but actually implementing it may not be so easy, involving scripts. The easiest would be to simply make an alias of the file so you are working off a single copy on the hard drive. An alias on your computer makes a pointer the files actual location on the external drive. Or for that matter, if you always have this drive attached, why not simply open it from its location on the external drive. I have 4 drives attached to my computer and I can open a file on any of them.

My immediate thought would be to create an alias on the Mac to the actual file on the external drive. That isn't making two copies though.

My next thought was to see if you could set up a Folder Action or Automator script to do this. I can't get you started on that but something along the lines of what is done here: https://www.popsci.com/diy/mac-automation/

Unless you go with an alias, the lowest tech solution is to remember to copy the file to the other drive before you shut down your computer. As I stated in connection with another proposal, you have to remember to do that. ;-)

Not possible unfortunately.

And then remember to run it before disconnecting the drive. ;-)