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.