How can I achieve the following behavior?

I have a simple performance where part 1 is responding to FC1 (which controls expression 11) and part 2 is set to not respond to the same FC1. I've done this by setting the EXP receive SW 'on' for part 1 and 'off' for part 2. All works fine.

Now, what I would like to do is to still use the 2nd slider to control the overall volume of part 2. However, by disabling the EXP receive SW on part 2 the slider is also blocked. I thought the slider would control volume which is different from expression, right?

Obviously I'm trying to solve this the wrong way. Any tips are appreciated.

Sorry, I seemed to have missed your question.

When you opt out on responding to EXP, for Part 2, it will no longer respond to the physical Fader. You can, however, manually set the Volume by highlighting the on-screen slider using the Data Dial or by assigning directly the "Part Volume" parameter to another controller.

You could assign it to an Assignable Knob, to the Ribbon, to the Mod Wheel, etc.

Say for example you could assign Part 2's Part Volume to where you touch the Ribbon. Say, 0 volume when you touch it on the left side, through to 127, when you touch it on the right side.

Without knowing what is already assigned within your controller box (Source/Destination) it would not be possible to tell you exactly what to do.. We can tell you what to look for... In general...

...you want to set the PART VOLUME to 0, then select a Controller that will be responsible for its output volume according to its position. (The Fader is no longer able to control it because the EXP setting).
Press [EDIT]
Press [PART SELECT 2] to view its parameters

In the screen, you want "Common" (blue) > "Part Settings" > "General" > set the Part Volume to 0 (or the minimum volume you want from this sound) but before we can make the controller assignment you should search for what assignment to Part Volume already exist. You do so by...
Go to "Mod/Control" > "Control Assign"
Set the DISPLAY FILTER = "ALL" so you can view what Destinations that are already assigned to this Part.
There can be 16 possible Destinations per Part... You'll be viewing four per Page. Use the PAGE box to view any additional Destinations
You are looking to see if "VOLUME" (Part Volume) is already assigned to a controller (Source). Duplication of functions is generally best when you know about them, rather scratching one's head when the unexpected occurs. If Volume is already controlled by assignment, you'll want to know!

If clear, you can go ahead and assign a controller to the Part "Volume".
Go back to "Part Settings" > "General" > highlight the Part Volume "0" setting
Press the [CONTROL ASSIGN] button (located next to the OCTAVE buttons) which glows when you have highlighted an assignable parameter.
You will be instructed to move/select a Controller to work this parameter.

You will be taken directly to the "Mod/Control" > "Control Assign" where the assignment has been made and is ready for you to scale the response for your preference.

Hope that helps.