Hernan,

WinTrackBar is the closest equivalent among our Windows Forms controls to WebSlider.

WinTrackBar can show a single thumb at a time.  You might consider using two WinTrackBar controls, and using code to ensure that the range the two controls define is meaningful (such as that your "start" value is not greater than your "end" value).  The following article from our online help documentation of NetAdvantage for .NET 2009 Volume 1 should provide a starting point, since it shows how to keep the values of two WinTrackBar controls in sync with each other:
Synchronizing Two WinTrackBar Controls