They don't have to both be open, but it makes things easier.
If you know the full path to the workbook to be linked to then you can set
the formula up manually. For example, if you had 2 workbooks open and linked
to one named Book2.xls the formula might look like:
=[Book2]Sheet1!$A$1
but when Book2.xls gets closed, then the formula might turn into something
like:
='C:\Documents and Settings\aUserName\My Documents\[Book2.xls]Sheet1'!$A$1
If you know what it's going to look like when the other book is closed, you
can set up the formula to it manually, i.e. by typing it into the cell (or
even creating a formula in VBA code).