Open two instances of a markup file in VS2008

With a normal .cs files i can choose "Windows" -> "New Window", to work in the same file in two different locations. I prefer this feature to "Split", because it seems less 开发者_如何学Goerror phrone in my version of VS 2008 SP1. However when I'm working on markup files (.aspx, .ascx, .asp) the "New Window" is dimmed, does anyone know if the there is someway to enable "New Window" on markup files ?

BTW. ".txt" are supported just fine.

Ok, this is a Hack by far .. but ATM is my best solution.

Rename the extension of the file to .txt. then open two instances, by using Window => New Window, next rename back to the original extension. Bingo, you have two open instances of the markup file.