Use 1 slider as a Supercontrol for the 2 hidden positive and negative sliders
Let’s assume you already have made a working slider for the negative values, and another for the positive values.
1) Make these 2 sliders invisible to the user. Go to Advanced tab, and set each slider to Hide in Play Mode.
2) Create a 3rd slider. This is the one the user will operate. Go to Relationships tab and set it to SuperControl.
3) Press Subcontrols, where you will see a button Drag to Subcontrols. Press and hold this, then drag and drop first onto the negative slider. The slider will bounce up and down to acknowledge.
4) Once again drag and drop that button onto the slider with the positive values.
5) Press Back, then Options as Super.
6) Press Sequential.
If the above steps were performed correctly, operating the new slider from Min to Max will first move the negative slider, then the positive slider. You now have one slider for the whole range from negative, to 0, to positive. Now you just have to go to the slider Properties to set Display Zero, # of Ticks, and Display Min => Max.
This is an essential technique, as many common parameters have this two sided aspect of negative and positive, with 0 in the middle. For example: Pan, Gain, Tune, Transpose, etc. Hope that helps!