If I understand correctly...  the issue is that you can't operate the Assign Switch from the Remote Keyboard (MIDI Controller) unless the Part is "physically selected" (exclusive selection) on the MODX?

What Model Keyboard are you using as a Controller? Which of its Buttons/Controllers are you using to activate the Assign Switch?

This might help you get an answer. 

No actually, I'm trying to use MODX (V2.52) ASSIGN switch to activate the XA Element Control programmed in a Multi-Part Performance in Part 2.  The XA Control only works when I've selected PART 2, which doesn't allow me to play the other Parts simultaneously. 

The other duplicate thread seems to have more pertanent info for your situation.   This would happen if you have keyboard control turned off for Part 2.  You can send the assign CC from the external keyboard on Ch2 instead of using the switch if your MIDI keyboard can map a button to work.  Or take the notes for CH2 and map them in an extreme octave of the keyboard where your physical keys are not and leave keyboard control on for Part 2 while compensating with note pitch offsets to get the pitch back to the center.

 You can send the assign CC from the external keyboard on Ch2 instead of using the switch if your MIDI keyboard can map a button to work. 

Testing on my M8X shows that works.

1. Init Normal (AWM2)

2. Set Element 1 XA Control to A.Sw1 to ON

3. Set part 1 Keyboard Control to OFF

4. Play a note on the local keyboard - no sound

5. Play a note on the remote keyboard (MIDI-OX) - no sound

6. Use MIDI-OX to send CC86 with a value of 127 - default control number for assign switch 1 is 86 - value of 127 = ON

7. Play a note on the remote keyboard - normal sound

Using an external keyboard or device to toggle the ASSIGN state is next-level since it extends the flexibility of XA control.  Using the physical button on MODX/Montage, the switch applies to all Parts with keyboard control on.  It's either on or off for all.  Each Part cannot have an individual setting.  Using an external MIDI device each Part can have its own custom setting. Part 1's AS1 could be on while Part 2's AS1 could be off and so on.  When you select an individual Part the different states of the buttons will be reflected on the LEDs.

I can't reproduce it for parts with keyboard control ON.

Using an external MIDI device each Part can have its own custom setting. Part 1's AS1 could be on while Part 2's AS1 could be off and so on.

Can you post the steps that demonstrate that? My M8X tests show that sending CC86 doesn't have any affect on parts with keyboard control ON unless you send it on the MIDI I/O channel (default channel 1). And then it affects all parts that have keyboard control ON.

I was using keyboard control ON as the counterpoint to the individual control you get from external control (and in the OP's case the assumption is that kbd Ctrl is off given what's been conveyed thus far).

I was using keyboard control ON as the counterpoint to the individual control you get from external control 

I wasn't sure so wanted to test that using an external trigger doesn't work on individual parts (on non-midi channels) that have keyboard control ON.

Sending CC86 on a non-midi channel (e.g. 3) has no effect if keyboard control is ON for that part but it has the desired effect if control is OFF.

And sending CC86 on the midi channel affects all parts that have keyboard control ON.