Basically the validator should be working even on multiple tabs.
I created a JSBin example which you can try and see that the form is not submitted until the validation passes.
I assume that in your case you are loading the tabs with Ajax and they are not validated because the inputs does not exist into the DOM before the content is loaded.
the Telerik team
Join us on our journey to create the world's most complete HTML 5 UI Framework - download Kendo UI