The RadDock's Width and Height properties are designed to work only with values specified in Pixels. If values specified in percent are used in some scenarios this might work, in others it will not work - in one word the behavior of the dock is unpredictable.
Based on the info provided I have created a sample project that has two <div> elements inside a dock. One of them has Text inside, and in the other I have a RadTabStrip control. I don't seem to be reproducing your behavior on my side. The <div>s are rendered correctly and the whole content of the first <div> is visible (it is not covered by the second <div>). You can find the project attached to the thread.
Can you please open a new support ticket, and send us a simple running demo, where the problem can be reproduced? The project will give us a clearer picture of the problem, and will help us provide adequate solution.
the Telerik team