The original World of Darkness games were not designed as a unified whole, as a result, while the games share a common system, crossing over elements tends to require a highly involved Storyteller. In addition, the various supernatural entities of the World Of Darkness are fairly ignorant of each other, meaning that in canonical campaigns there should be minimal, if any interaction between the different groups - generally as a rare antagonist or a cameo. Crossover is used generically to refer to both mechanical or plot situations where characters from two different games interact. No Whitewolf publications provide any guidelines for crossovers between the games set in the World of Darkness.
Crossover problems are less important in the current World of Darkness since a common system was designed from the start.