How to do simultaneous fitting to the mulit-variables

Dear RooFit expert,
I have a very simple question.
In my analysis, I have a control region, which include ttbar, w+jet, QCD background, I want to do a fitting
to determinate the conponents of each background. I selected a few variables to do the fitting simultanous, for example, transfer mass, letpon eta … I have tried to use RooSimultaneous to , but unfortunatelly, I have
a problem when construct combined dataset RooDataSet.
Are there any example to do this kind of jobs. Many thanks