You can filter the DOSE-EQ USRBIN scoring as a function of the parent radioactive isotope, whose identity (in the form A + 1000 Z (+ 1000000 m)) is stored by the IAZTRK variable in trackr.inc.
This means that you can clone the aforementioned USRBIN and associate each copy with a different contributor. Such an association has to be made in the fluscw.f user routine (requiring the addition of the USERWEIG card), by including trackr.inc and setting LSCZER to .TRUE. if JSCRNG - i.e., the index of the USRBIN copy - has a given value and IAZTRK does not correspond to the respective nuclide.
I did not test the above, so we’ll be happy to follow up in case of issues.