When you add a few DocumentWindows to RadDock and you undock one of them to a floating window, then undock another one in the same floating window you will see two tabs. In the main area the selected tab is bolded, the functionality should be the same in the floating windows

For the time being use the following code to manually bold the tabs:

this.RadDock.FloatingWindowCreated += RadDock_FloatingWindowCreated;
...
void RadDock_FloatingWindowCreated(object sender, FloatingWindowEventArgs e)
{
    e.Window.Controls[0].ControlAdded += Form1_ControlAdded;
}

void Form1_ControlAdded(object sender, ControlEventArgs e)
{
    DocumentTabStrip tabStrip = e.Control as DocumentTabStrip;
    tabStrip.ControlAdded -= Form1_ControlAdded;
    if (tabStrip != null)
    {
        tabStrip.SelectedIndexChanged += tabStrip_SelectedIndexChanged;
    }
}

void tabStrip_SelectedIndexChanged(object sender, EventArgs e)
{
    DocumentTabStrip tabStrip = sender as DocumentTabStrip;
    if (tabStrip != null)
    {
        foreach (DocumentWindow item in tabStrip.Controls)
        {
            item.TabStripItem.ResetValue(RadItem.FontProperty, ValueResetFlags.Local);
        }

        tabStrip.SelectedTab.TabStripItem.Font = new Font(tabStrip.SelectedTab.Font.FontFamily, tabStrip.SelectedTab.Font.Size, FontStyle.Bold);
    }
}