If a crossover is one-time only, it's easier to write off.
If, however, two shows have regular crossovers with each other, such as Homicide and the various Law & Orders, then that does indeed place them in the same universe. I'm not aware of two shows that do this as often as L&O/Homicide did, though.
If, however, two shows have regular crossovers with each other, such as Homicide and the various Law & Orders, then that does indeed place them in the same universe. I'm not aware of two shows that do this as often as L&O/Homicide did, though.