King Air is a single pilot aircraft, but most operators find it is not possible to get affordable insurance for single pilot public transport operations. It turns out cheaper to pay an F/O. This F/O will need to be type rated.
On positioning flights and/or private operations there is nothing stopping you from sitting in the right hand seat without a type rating and if you have a valid FRTO licence you can even do the radio, however unless the guy in the other seat has a valid instructor rating on type you cannot log the time.
Regards
Gerard