My office mate is a binary star person and they can get the individual
masses. Here is roughly what they do (this may be a little garbled as I'm
trying to remember a 5 minute lesson my office mate just gave me). (Details
not included, nor batteries)
Basically they use Kepler's 3rd law to get the mass sum. This comes from
both photometric and spectroscopic data.
Then they get the mass ratio from spectroscopic data alone.
For non-visual binaries you need to have an eclipsing binary star system for
the mass sum (for non-visual binaries), visual binaries are easier because
you just watch them and if you know the distance to them you can easily fit
the Kepler law parameters to find the mass sum.
Spectoscopic measurements give them the radial velocities of the two
components.
Photometric measurements (how much does the intensity decrease with the
eclipsing for the two components).