How to sync two directory on one device.
Such processes are local synchronizations. These are only feasible with a further instance.
Maybe there will be a correspondingly integrated feature, which in my opinion would be very desirable, as such questions arise frequently. A similar question of selective sync is asked just as often, and here too the wish would be similar.
As said above, the only way to do this is to run multiple instances. There are drawbacks to this approach, e.g. double the resource usage, unnecessary data encryption and decryption, etc.
Unless you need specific Syncthing features, there is usually other software that can do the same job faster, with lower resource footprint, and also without having to hack around in the first place.
That said, I’ve been using Syncthing locally myself, but this is mostly to have a local pseudo-backup relying on versioning.
FreeFileSync does this on demand and in the background (RealTimeSync). Available for macOS, Linux, and Windows.
You can not compare a desktop solution with a P2P solution, is very different in my point of view.
A P2P solution is not a good fit for what the OP wants to solve; he needs a desktop solution, I suggested one. I should have made that more clear.