Hello,
I have 7 instances of the same user web control (.ascx) nested inside RadMultiPage - each one is inside different RadPageView, of course.
Strange thing is that even RenderSeletedPageOnly is set to true, all user web controls placed inside radpageviews are somehow initialized on server side and that caused be this problem with unique keys and some another problems with saving default state, etc...
I have to load user control programatically in OnInit page method. There I check which radtabstrip is actually selected and then only current one user contorl is loaded. This way it works as expected...
Best regards
Vasssek