The 12V switched source would be what lets the radio play, so it turns on when the ignition is turned on (if the radio switch is on). The constant power 12V source would be what keeps the preset stations in memory, although not all radios lose them when power is cut.
You could accomplish that with your single converter by adding a relay to the ignition switched 6V circuit that closes when key is on to power the switched 12V. Pull two wires off the converter (12V side), running one to the constant 12V input and the other thru the relay to the switched 12V input on the radio.
If you didn't wire it that way, but just sent constant power to both 12V inputs, you'd have to turn off the radio every time you shut off the truck. I think it's mainly a convenience thing.
Last edited by klhansen; 04/11/2020 8:47 PM. Reason: added more info.