How do I get a code window to split vertically in Visual Studio 2008? (not HTML mode)

In Visual Studio 2008, it seems that Window -> Split only gives me a horizontal split, and Window -> New horizontal tab group does not allow me to edit the same file in both panes.

Note that I am talking about C# code windows, not HTML editing mode.

I want to edit the same file in two horizontally arranged panes.

Is this possible? Free add-ons are fine.

Edit: I have CodeRush Express installed, and it seems to have done something to my code window borders.

Edit #2: It's not CodeRush. I tried on VM with no CodeRush, and I have exactly the same issue.