Same grounding, understand at that point it becomes a common ground point system, and the only thing different between the two terminals is voltage potential. Say, for example, if you have a 60V source to ground and a 160V source to the same ground, each one has their own respective voltage potential to ground, but if you, for example, connect a light bulb between the 60V and 160V terminals, it would only have a voltage potential of 100V, verses 160V or 60V. That is voltage potential difference.
As for common grounds, it just makes things simpler.