Is it safe to lower the gas on a dual ring burner so that only the inner ring is lit?

This is almost certainly normal and how the ring is supposed to work, and it should be perfectly safe to use it this way as the gas is cut off when you turn the ring down. I have these types of ring myself, there can occasionally be a whiff of unburned gas when you turn it down past the single ring threshold and the outer ring cuts off, it's just a hint for a second and then it disappears.

The best way to tell if it is safe is to use your nose, mercaptan is added to natural gas to make it smell offensive, and your nose can detect trace amounts of it. If you don't smell any gas with the outer ring off then there isn't any unburned gas.