My experiance as a cyclist and a motorist would advise against it. Most of Highway 1 along the Mendicino coast is narrow, winding and traffic moves at a fast pace and there are many sections with no shoulder.
Most of the roads going inland from Highway 1 are also narrow, steep, winding and with little or no shoulder. I'm not saying this to discourage tandem rides on the coast, but just to warn about the conditions.
Jim McCoin
Fremont Ca.