You need to use FindControl() like with any other nested control (e.g., a Panel). Here is an example:
protected void Page_Load(object sender, EventArgs e)
{
RadDropDownList rddl = RadPageView2.FindControl("RadDropDownList1") as RadDropDownList;
rddl.Visible = false;
}
assuming your structure looks like this (whether it is generated from the code-behind is irrelevant, you only need to recreate it early enough):
<telerik:RadTabStrip runat="server" ID="RadTabStrip1" MultiPageID="RadMultiPage1">
<Tabs>
<telerik:RadTab Text="first"></telerik:RadTab>
<telerik:RadTab Text="second"></telerik:RadTab>
</Tabs>
</telerik:RadTabStrip>
<telerik:RadMultiPage runat="server" ID="RadMultiPage1">
<telerik:RadPageView ID="RadPageView1" runat="server" Width="100%">
PageView
</telerik:RadPageView>
<telerik:RadPageView ID="RadPageView2" runat="server" Width="100%">
<telerik:RadDropDownList runat="server" ID="RadDropDownList1"></telerik:RadDropDownList>
</telerik:RadPageView>
</telerik:RadMultiPage>