You can easily achieve your goal by setting the RenderSelectedPageOnly property of RadMultiPage control to true. Please note that in order to use the RenderSelectedPageOnly property of the multipage, you will have to set the AutoPostBack property of the tabstrip to True as well. In addition, if you want to avoid the postbacks while moving between tabs, you can ajaxify both controls; simple place the tabstrip and the multipage controls into a RadAjaxPanel.
Still, have in mind that RadTabstrip always reloads all of its pageviews. The RenderSelectedPageOnly property does just that - it renders only the selected page view; the code-behind of your page / UserControls will be fired no matter which pageview is rendered. Loading all pageviews is essential for the server controls living in the page views. If we don't load them every time viewstate will be corrupted and postback events won't fire.
Alternatively, you can use the approach shown in our TabStrip / Load on Demand RadPageView example.
All the best,
the Telerik team