how do you make the distinction between the two in your practice?
I will try to outline how I learned it:
We se hara as bigger region ~ between pelvis and diaphragm: The belly. The tanden is a certain part of it which is located inside the hara.
The hara is more or less directly connected to the hips, the pelvis and chest. So they move "together": Hara, koshi, hips, pelvsis is one unit so to speak. You can move the hara by moving the hips, koshi, pelvis and maybe the legs. The tanden is freely moving inside the hara. So it can rotate. And it's movement is independent from pelvis, hips, koshi.
Little bit Like a coconat swimming in a fishbowl …
In our practice the outer movements of the body relate to the hara: Direction of the hips, stance, legs, moving forward, … The hara relates to the movements you can see.
The "inner movements" like guiding the kimochi, sending "strength" or "impulse" through the body without outer movement relate to the tanden.
Did this help (even if you don't agree with this distinction)?
I think most people are speaking of hara and tanden as synonym. That's why I asked for it.