There is no problem to nest a user control directly in a separate RadToolTip. What could be causing the problem in your case is the following:
1) You have provided incorrect TargetControlID and thus teh tooltip show is never triggered.
2) You are using the obsolete properties Sticky and ManualClose which are logically not possible to be used in combination - please remove them and use the HideEvent instead.
In case this does not help, please provide more details and source code and best - a live urll where we can observe the problem.
the Telerik team