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Touchable slider with two sliders - best practice

I'm looking for a userfriendly solution to change two values by only one slider. enter image description here
I have an upper and a lower threshold whereas each value is represented by a knob on the slider. As I'm dealing with touchscreens, each knob has a clickable-region. If the knobs are close to each other, thus overlapping regions, the user will always select the knob layered on top. The only way I could think of solving this would be allowing the user to click again and then selecting the other knob.
Assume the following situation:
1) User shifts the left knob close to the right one
2) User wants to shift the right button even further right but he will grab the left one because this was layered on top by default.
3) User has to click again on the region to select the other button
4) After all this, the user can press again and drag. The other solution would be to start dragging as soon as the second click was performed.

Another solution would be to place some radio buttons close by and select the slider.
Another issue is of course the fine adjustment of the sliders, thus I need to place some +/- button somewhere. I'd like to just place one set of +/- not for both, which would lead to always have one knob active.


Is there any guideline out there which addresses such an issue?