There really isn't a user experience benefit of letting sliders overlap. The right side of the left slider should not be able to be dragged past the left side of the right slider.

You must subtract the pixels that each handle occupies on the slider, as that space really isn't part of the slider. 50% for the right slider would be exactly 1 slider width off from 50% of the left slider. This way, if you wanted to select exactly 50% then the two sliders would be next to each other, not over the top of each other.

This is a much better user experience and is the way sliders in every other slider example I can find work.

Example:

http://developer.yahoo.com/yui/examples/slider/slider_dual_with_highlight.html

Note that the left slider can never ever even slightly overlap the right slider.