Three Signals

Profibus.

...or an additional multicore cable.

If you want to use a single pair cable for this application, you will need to transmit your 24VDC as a baseband signal on the cable, then modulate the 4-20mA and feedback signals onto the cable and de-modulate those signals at the remote end. I'd recommend wave division multiplexing. All of this will be a lot more expensive than simply running a 3-pair cable like PWSlack said in #2.

A HART analog output could drive the positioner (95% of positioners are 2 wire that derive their power from the 4-20mA control signal) AND provide position feedback in a HART enabled system. I assume Profibus PA, mentioned above, can do the same.

Screen shots from Fisher Fieldvue HART positioner manual, showing two wire 4-20mA HART signal supports reading the "travel" variable that equates to position.

You really have only two functions over the wires.

The 24V is not power for the positioner, it moves a small pneumatic pilot valve which then moves the valve trim pneumatically. The hardware interprets 20mA as a command for one extreme of trim movement, 4 mA the opposite. The voltage drop at currents above 3mA powers the HART signal processing hardware (modem), providing 2 way communication to the valve positioner. Open commands and position feedback, alarms, maintenance mode, calibration routines can be sent across the HART signal pair, though most engineers prefer the 4-20mA signal directly operate the valve position. HART modem signals in heavy industrial environments can have reliability problems if raceway design is sub par.

I think he is asking about an existing process... like he does not understand how 3 different signals can be transmitted across just 2 wires. But I could be wrong.

Yes, there are very few possible options that fit the limited information provided. HART is the more popular solution that fits with only two wires connected. I think Profibus might require a power source in addition to the communication media, but that technology could have advanced since I’ve checked in on it.

Another possibility:

Foundation Fieldbus (FF) is like HART or Profibus PA - DC power is carried on the same two conductors in twisted shielded pair cable as the signal. Multiple variables are available through FF just like HART or Profibus PA.