If you have two separate projects that is somehow connected. How can one make a reference to the source of the other project?

For referencing the source of your own project you use:

source:some/file

But since I want to refer to code in another project my thought was that I could write something like:

other_project:source:some/file

Anyone that knows if this is possible in some way? I have read http://www.redmine.org/wiki/redmine/RedmineTextFormatting#Redmine-links but found no clues there.