FreeFileSync Forum

Your proposal would require two instances of FFS, each running on respectively the left- and right location (for a sync job that has just a single left-right pair).
If you are syncing between two computers you have full control over, that would theoretically be an option.
However, most of the time ...

That wouldn't be any faster than shared folders. I am using shared folders at over 1 gigabyte per second and have no issue using FFS or native File Explorer.
I'm glad that you have such a nice connection, but not everyone have it.
For me remote scans are like 100+ times slower (mostly due to ...

I mean not through shared folder, as it is very slow to scan, but between 2 FreeFileSync applications?

Each act as local folders scanner and then they compare results between each other.
That would be many times faster.