How to detect a change in resistance?
I can't think of any easy way to do it without a microcontroller.

Find a small microcontroller that has (at least) two ADC channels, and use these to measure the two real resistors (potentiometers, rotary encoders, whatever). Use firmware to decide which is "most recently changed". Drive a digital potentiometer from the microcontroller to operate the actual circuit.

This is a tiny, two-chip solution. Anything else would require many more parts.