Solved!
The problem is that you have to feed the ingredient pdfs of the RooSimultaneous to the RooSimPdfBuilder constructor e.g:
RooSimPdfBuilder mgr(DC, gauss_1, gauss_2.....) ;
Not just the RooSimultaneous itself e.g:
RooSimPdfBuilder mgr(simPdf) ;
The solution was found here:
root.cern.ch/root/html510/src/R … r.cxx.html
Thanks to those that replied!
S