There'd never be a crossover. Even though the shows had similar themes and often really similar story-lines, they were actually rivals. Crossovers, other than fan fiction ones, only happen when the shows are on the same network or are written by the same people. Normally they happen when one show is a spin-off from the other.

Also, all of your original answers were ridiculously biased. None of them expressed the impracticality of a crossover, or the possibility that someone could actually not like the idea of a crossover, or might not like both shows.

Well there's a first time for every thing! They might have done a crossover if both shows agreed! And btw Buffy the vampire slayer and Charmed used to be on the same network the WB until they changed their name to the CW!