Slider for a trade-off between two options

Hi everyone,
I would like to use a slider to allow subjects to make a trade-off between two options. Specifically, I want to let them assign a probability to both options which will add up to 100% (e.g. 70% of getting option X and 30% chance of getting option Y). Something along the lines of the screenshot below.
I have seen solutions to getting labels at both ends of the slider. But I am struggling with how to remove the values above the slider and showing the probabilities of the two options (below the slider or somewhere else) while the subjects are moving the slider bar.
I have some basic understanding of JS, but I find it hard to find a starting point. If anyone can point me into the right direction that would be greatly appreciated!