I'm trying to create a TrackBar with two sliders.
I thought about how I would implement such a thing, and the attached test program and unit might give you some ideas.
I implemented mouse and keyboard support (but not design support if you want to turn it into a standalone component).
With two sliders there are various behaviours that you can support. In the attached example clicking on the track affects the lower slider if you click to the left of it, and clicking between the sliders causes them to move towards each other. Clicking to the right of the higher slider moves it further right.
Keyboard movement with arrow keys affects the lower slider. To move the upper slider, press Ctrl+arrow key.
I have not implemented working code for the vertical orientation, but left placeholders you can complete if this is what you need. It is almost untested, and only so far on Linux, so I am sure it has bugs. It is offered as a proof of concept.