Shoot a whole film on a single scene "at once" (all 36 frames). If possible put your camera on a tripod and use a camera with motorwind. Take a scene with some contrast & details. Preferably under circumstances with a steady lighting during shooting (avoid days where the sun comes & goes because of clouds, etc.).
Then cut the exposed film in half and develop each part separately (with & without added salt, or whatever the factor is) to see what the effects are. Keep all the other factors (temp, agitating, developing times, ....) identical to eliminate a combination of effects. This way you're certain what factor causes what deviations in results.
Can you see any difference? And do you like it?