In Dwarf Fortress, cats choose their own owners. Once a cat does so, killing this cat will piss off the owner, who, being a completely insane DF dwarf to begin with, will promptly throw a tantrum which will probably end up killing half your fortress. The other feature of cats is that they breed insanely fast. If you bring two cats during embarking, they will automatically be a mating pair, which will proceed to mate like crazy and drown your fortress in cats that you cannot kill due to aforementioned tantrum reasons but still do nothing but lag the fuck out of you. The solution for that is caging cats before they adopt an owner, and sending them to the butcheries.
Embarking with two cats is not a good idea for this reason, hence 2cat.