Re: 1 Slider, 2 Attributes

When you're trying to add math to commands you are often required to put it in parentheses. So this might work:
else setAttr "BodyBlendShape.ConvexNoseBS" (-1 * $mySliderPos);

Another way to get rid of a negative number is with the abs() function. abs($mySliderPos) will always return a positive value.

Remember you're still going to need to add some extra code to zero the unwanted blendshape in both cases, since I assume you don't want them to overlap and add to one another.

Hope that helps,

-Steve