Can I analyze disconnected structures in one model? Can I trust the results from this model?

Yes. STAAD.Pro can analyze two independent disconnected structures and results can be trusted. It would show a warning message reporting multiple disjointed structures to be present. However this should not be done when one is attempting seismic load generation ( equivalent static lateral force method ) or dynamic analysis like response spectrum or time history. In general, if structures that are to be analyzed, are independent, the best practice is to have separate models for each.