If a TabStrip with two editors is configured, the editor in the first tab is working correctly, but if the second is shown - its toolbar is not working as expected.

Possible resolution is to insert a meta tag in the head element of the main page, which sets the rendering engine of IE10:
<meta http-equiv="x-ua-compatible" content="IE=10" />

Another possible resolution is to call the onParentNodeChanged() method after the tab is selected:

<telerik:RadTabStrip ID="RadTabStrip1" runat="server" SelectedIndex="0"
    MultiPageID="RadMultiPage1"
    OnClientTabSelected="OnClientTabSelected">
    <Tabs>
        <telerik:RadTab runat="server" PageViewID="RadPageView1" Selected="True" Text="Root RadTab1">
        </telerik:RadTab>
        <telerik:RadTab runat="server" PageViewID="RadPageView2" Text="Root RadTab2">
        </telerik:RadTab>
    </Tabs>
</telerik:RadTabStrip>
<telerik:RadMultiPage ID="RadMultiPage1" runat="server">
    <telerik:RadPageView ID="RadPageView1" runat="server" Width="100%" Selected="True">
        <telerik:RadEditor ID="RadEditor1" runat="server">
            <Content>
                Content Page 1
            </Content>
            <TrackChangesSettings CanAcceptTrackChanges="False" />
        </telerik:RadEditor>
    </telerik:RadPageView>
    <telerik:RadPageView ID="RadPageView2" runat="server" Width="100%">
        <telerik:RadEditor ID="RadEditor2" runat="server">
            <Content>
                Content Page 2
            </Content>
            <TrackChangesSettings CanAcceptTrackChanges="False" />
        </telerik:RadEditor>
    </telerik:RadPageView>
</telerik:RadMultiPage>

<script type="text/javascript">
    function OnClientTabSelected(sender, args) {
        var tabIndex = args.get_tab().get_index();
        if (tabIndex === 1) {
            var editor = $find("<%= RadEditor2.ClientID %>");
            editor.onParentNodeChanged();
        }
    }
</script>