How to set the value on a slider through Javascript

Hello,
I am trying to have two sliders in a question which automatically update one another. If I change the value of the first slider to x, the second slider should update to 500-x. My current code is:
var currentQuestionInfo = this.getQuestionInfo()
var postTag = currentQuestionInfo.postTag
var currentQuestionID = this.getQuestionInfo().QuestionID;
var Sli1Old = 250;
var Sli2Old = 250;
var DomSli = 0;
var respSli1 = document.getElementById(currentQuestionID + '~1~result');
var respSli2 = document.getElementById(currentQuestionID + '~2~result');
    
Event.observe(document, 'mouseup',function(e){
   var currentResponse1 = respSli1.value;
    var currentResponse2 = respSli2.value;
    if( currentResponse1 != Sli1Old ) {
       DomSli = 1;
    } else {
        DomSli = 2;
    }
    if( DomSli == 1) {
        try {
            var forceAns2 = 500 - currentResponse1;
            this.setChoiceValue(2, forceAns2);
        }
        catch(e) {
        }
    }
    var currentResponse1 = respSli1.value;
    var currentResponse2 = respSli2.value;
    Sli1Old = currentResponse1;
    Sli2Old = currentResponse2;
});
All the variables work and the current responses are read out correctly. However, the setChoiceValue function does not work. Any idea on how to fix this?
Thanks in advance!