Hmmm....
This seems pretty complicated to have two forms.
A couple thoughts. You have both open concurrently you could maybe use code to pass the file name and path back to the originating form so it could be saved.
Probably easier is to just query the 2007 library with the 2003 form and use some common data to search the library, find the cooresponding form, and display the link to that form in the 2003 document.
If you used a date / time stamp.
Save a stamp in 2003 when they open a blank 2007 form and the user name of the user.
Save a stamp and username in the 2007 form upon open and promote these fields in the library.
You would have what you needed to link these two.
Same username and stamps within a second or less.