I'm not real familiar with the Plymoth business coupes, but would venture to say they probabably followed the other two. Business coupes had smaller doors (sedan doors) and the two seaters had larger doors. Puting a rear seat in is not difficult, but getting your passengers in and out could be a challange. Compare your door size with the two seat coupe before jumping in. Can still be done if you install something like the early extended cab PU seats that move very far forward to grant access to the rear area.
just adding to what Trees said, you could get a seat something like what Jeeps use, they hinge forward to get in the back. something else that would be totally wicked if you're really into customizing, would be to do something like a 3rd door like on the new Saturns and pickups.