What I normally do is copy the drawing with a new name. Then when you click open and browse to the copied drawing and select it, there is a button in the open dialog box called "References..." from there you can select a new reference to the other part/assembly.
Hope this makes sense!
Best Regards,
Jon
Challenges are what makes life interesting; overcoming them is what makes life meaningful.
Solidworks 2005 SP1.1