How can I access controls inside a RadWindow (defined as UserControl) from another usercontrol? Is this a parent/child issue?
And another question: Is it possible to load a RadWindow (defined as UserControl) directly into a second monitor if my desktop is extended?

Hello Roy,

For a sample project about your first questions you can refer to this forum post: http://www.telerik.com/community/forums/silverlight/window/how-to-raise-event-for-my-scenario-using-telerik-s-rad-window.aspx. In it is illustrated how to access one RadWindow from another. You can use the same approach to achieve your goal.
As for the second question: unfortunately, this is not a supported scenario by Silverlight. You cannot show a Silverlight RadWindow outside the Silverlight plug-in.

Hope this information helps. If you have further questions please let us know.

Kind regards,
Konstantina
the Telerik team