Hello Slavisa Radicevic,
No, unfortunately there is no way to load the visual tree for content that is not currently selected, since switching between tabs makes the TabControl unload the content of the last active item and load the content of the newly selected item. This is done for performance reasons mainly.
However, in your case I can suggest you to databind the UI elements which content has to be validated. This way you will be able to access the properties the elements are bound to at any time and validate all necessary info.
Give it a try and let me know if this approach works for you or if you need more info.
On a side note, I'd like to inform you that we've just released an online tool that allows you to reduce the size of your Silverilght applications. For more information, please visit http://blogs.telerik.com/blogs/posts/10-06-10/telerik_assembly_minifier.aspx
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