Your second slider could disappear in case there is a JS error, because the HTML of the slider control is created on the client. That is why, I suppose this is the problem in your case. In case you could send me your code, I will be able to check this on my side and find the cause for the problem. Generally, the current version of the RadSlider has one problem with the server ValueChanged event - this event is fired before Page_Load and the problem could be connected to this. This problem is already fixed and the fix will be available with the Futures release, scheduled for this Thursday.
However, as I am not certain that this is causing the problem you describe, it would be best if you could send me your code, so that I can research it.
the Telerik team