The RadSlider supports the "increase" and "decrease" handles, but "decrease" only decrements SelectionStart, and "increase" only increments SelectionEnd. Ideally there'd be an option for *two* sets of buttons at each end of the slider, an "increase" and "decrease" pair, so that both SelectionStart and SelectionEnd could be *either* increased *or* decreased independently. See the attached illustration. Thanks! 

There is IncreaseDecreaseHandlesOnBothEnds SDK demo available since R3 2016 Release.
https://github.com/telerik/xaml-sdk/tree/master/Slider/IncreaseDecreaseHandlesOnBothEnds