If it's the same PA31 or C210 i'm thinking of then it's a multi-engine class rating that's required unless it falls under HPA (High Performance Aircraft) due to pressurisation. If it's a MEP class rating then 6h of MEP training,a test and written exam on a piston twin are all that's required, but it would of course very sensible to be checked out on the particular make and model by an experienced instructor!
I'm pretty sure you do not need a 'type rating' for these aircraft types.