We offer two different suites - RadControls for ASP.NET and RadControls for ASP.NET AJAX (formerly known as "Prometheus"). When using RadControls for ASP.NET you need to use a different dll for each control - e.g. RadSplitter.Net2.dll, RadComboBox.Net2.dll, etc. When using RadControls for ASP.NET AJAX you need to use only a single dll - Telerik.Web.UI.dll - which contains the code for all the controls.
The RadControls for ASP.NET AJAX suite includes a few new controls which are not part of the RadControls for ASP.NET and RadToolTip is one of them. This being said there isn't RadToolTip.Net2.dll file, but in order to use the RadToolTip control you should upgrade to RadControls for ASP.NET AJAX. We strongly recommend this upgrade not because you will be able to use the new controls, but also because they are improved, work better and they are based upon the MS AJAX Framenwork.
the Telerik team