Ok i did follow the second suggestion which is adding the tooltip to RadAjaxManager and then set the text property from code behind.
e.g.
The Buttons and The ToolTips are well formed and they actually work like a charm but, not entirely.
When i click domain checking button it works fine. Then i click username checking button and it works perfect too. But, if i click the domain checking button again it displays the Username tooptip :(
From this point it displays ONLY the Username tooltip. Domain tooltip is not showing up anymore.
Any ideas? Thanks
EDIT: actually nothing works after you display these tooltips. Neither validation nor the other buttons. Whatever you do after that the Username checking tooltip is displayed again and again. Is this sort of bug or i am missing something? Thanks