Simoultaneous fit with RooFit


I am new to RooFit so sorry if my question is too trivial.

For my analysis I need to fit the number of events in two different datasets A and B. Each dataset has its own PDF (PDF_A and PDF_B). For the fit I would like to require that the fraction of the number of events fitted in sample A ( N_A ) and sample B (N_B) is a constant i.e.:
N_A/N_B = constant

Is there a simple way to implement this with RooFit ?

Thanks in advance,