Thank you for your answer, it was very useful, I succeeded.
I just would like to mention that it's necessary to change:
RadMultiPage mp = (RadMultiPage)Page.FindControl("RadMultiPage2");
RadMultiPage mp = (RadMultiPage)this.Parent.Parent.Parent.FindControl("RadMultiPage2");
that is to say, change Page for this.Parent.Parent.Parent.
if not there's a exception failure of the type 'System.Web.HttpUnhandledException'.