0 Not sure whether it is possible. But you can do a workaround that to use another user control inside user control.
http://www.dotnetcurry.com/ShowArticle.aspx?ID=155
http://aspboss.blogspot.sg/2010/09/how-to-access-one-usercontrol-from.html
https://www.codeproject.com/Articles/29674/How-To-Access-a-User-Control-from-Another-User-Con