function validateSliders(){
var e = document.getElementById("frmMap:redSliderInput");
alert(e.value);
}

Surfing to find something else I come across this

https://labs.jboss.com/file-access/default/members/jbossrichfaces/freezone/docs/devguide/en/html/FAQ.html

5.23. How to retrieve the current value from the inputNumberSlider?

take a look[/url]

I didn't tested but it will goes like this

function validateSliders(){
var first = document.getElementById("frmMap:redSliderInput");
var second = document.getElementById("frmMap:redAnotherSliderInput");
second.value=first.value
}

Have you try to rerender entire component them after javascript action (onchange) is call.

something like this (haven't tested)

<rich:inputNumberSlider width="500" step="20"
 minValue="0"
 maxValue="500"
 value="0"
 showInput="false"
 showToolTip="false"
 showBoundaryValues="false">
<a:support event="onchange" reRender="anotherSlider">
<a:actionparam value="#{bean.firstSliderValue}" assignTo="#{bean.anotherSliderValue}" />
 </a:support>
</r:inputNumberSlider>