WPF TabControl Memory issues

Question

If I have the follow TabControl and everything works properly and when I switch to the second tabitem it just shows without no issues.

<TabControl>
   <TabItem> //datagrid </TabItem>
   <TabItem> //datagrid2 </TabItem>
<TabControl>

But if i have this xaml, when i click the second tabitem the memory goes out of control

<TabControl>
   <TabItem> //datagrid </TabItem>
<TabControl>
<TabControl>
   <TabItem> //datagrid2 </TabItem>
<TabControl>

What am I missing?