If you want to have http://yxyx.com/u/tiju you should set it in the HyperLink
<asp:HyperLink ... NavigateUrl='/u/tiju' ...
In this case rewrite rule could be as
<rewrite url="~/u/(.+)" to="~/Profil.aspx?user_name=$1" />
As you see, in this case you will send only one parameter (tiju)