Apparently, when the impact hypothesis was first presented as more than just speculation (i.e. with the iridium layer as evidence), a majority of paleontologists rejected it, preferring one based on long-term high-intensity volcanism in Asia. I believe this hypothesis still has a few proponents.

The collision of comet Shoemaker-Levy with Jupiter occurred as several distinct impacts, due to gravitational disruption of the comet in a previous close encounter with the planet. Something similar is clearly possible on Earth (perhaps even gravitational dispersion of a comet passing close to the Moon just before impact?) but the Chicxulub crater is large enough to explain the extinction event by itself (unless you hew to the volcanic explanation, I suppose), so the question is whether there is any evidence for multiple impacts. As far as I know, the global distribution of the physical evidence is indicative of a single source, but not definitively so.