The RadSplitter control would only resize if it is visible. That is the reason why the other(invisible) splitters are not resizing.
From your description of the problem that you are experiencing, I understand that you are using the RadSplitter only for the purpose of aligning the OK/Cancel buttons at the bottom part of the RadWindow.
If that is the case and you insist on using the splitter, then you should find a way to capture the moment a splitter becomes visible, i.e. when a tab is selected, and force the splitter to resize then. The OnClientTabSelected event appears like the best choice for that purpose. Then making sure that you have the correct dimensions of the splitter you can invoke its resize method. Otherwise I would suggest that you do not use the splitter and implement a CSS only approach for aligning the buttons at the bottom part of the window.
Hope this helps.
the Telerik team