PDF with 2 observables(x,y) and requirement y > x

I have a self constructed PDF which describes the decay of a parent nuclei and a subsequent daugther nuclei.
Each datapoint has 2 observables x and y, while x is the time of the first observed decay and y is the time of the second observed decay of the nuclei. Therefore the requirement is : 0 < x < y < upper time limit. How do i include that requirement (which changes obviously for each event) if I wanna generate a dataset from that model?

// decayevent2 is the PDF class
 decayevent2* roofit_decayevents2 = new decayevent2("roofit_decay2","roofit_decay2",x,y,*par[1],*par[2]);

RooDataSet* data2_sim=roofit_decayevents2->generate(RooArgSet(x,y),10000);


I suppose that you have to express the PDF in a way that is = 0 if y <= x . And you will use standard bounds
[0, upper limit] on the variables x and y.

Best Regards