Selection efficiency from invariant mass fit in RooFit

Hello,
I am looking for dilepton decays of a resonance. The selected sample I use for my analysis is composed by two tight leptons (where “tight” could be perhaps represented with a RooCategory). I would like to evaluate, from an invariant mass fit, the number of tight+loose events, in order to have an estimate of the efficiency of my “tight lepton cut”. (here by “loose” I mean a lepton which does NOT pass the “tight” selection)
How can I achieve this with RooFit? Actually I could manually shift the tight+loose sample of enough GeV’s to be able to separate the samples thus maintaining the same parameters for the two signal pdf’s (an extended likelihood fit could possibly do the job and give me the number of tight+loose events over the tight+any sample), but perhaps there is a proper way to do this.

Thank you in advance!

I didn’t spot a sort of solution (together with its plotting problem) were yet available on http://root.cern.ch/phpBB3//viewtopic.php?f=15&t=11936.