On Friday, September 7, 2018 at 5:39:17 PM UTC-7, wrote:
I have such a radio, and when you turn one of those concentric knobs, the other one turns too, unless you hold it firmly. Thus it takes two hands to turn only one knob. How can I adjust it work as intended?

On most dual concentric knobs, you take off the cap of the smaller (closer) knob and loosen the screw under the cap. Then pull the knob out a bit and tighten the screw. Check it for friction, make sure it's really tight and put the cap back on.
Jim