MC could do this, in theory, but you probably don't want to go down that route.
It would involve importing both source and backup files to your library, and then 'stacking' them so that you would only ever see the source file, but tagging would propogate down through the stack.
You will light up reading that, thinking (probably) 'perfect, exactly what I need', but, I would advise against it quite strongly. You would want to be 100% sure that the source files were all 'top of stack', and remember to go through this every time you add new photos. All frought with danger, in my humble opinion.
Perhaps a better workflow...
If you look at Tools > options > library & folders > manage library fields, you will open the MC Field Manager.
If you select a field, you will see a check box in the options on the right called "Save in file tags (when possible)".
For the image tags that are important to you, make sure that is checked.
When you're done with that, select all images and right click > Library Tools > Update tags from library. This will write any fields checked in the previous step to the actual file.
You then need to find a 3rd party sync tool to keep the source and backup folders in sync. I use GoodSync, but there are free alternatives out there, one that gets many mentions here is, IIRC, called "FreeFileSync"
-marko