Several years ago, we rode our tandem from Monterey to Santa Monica via Highway 1 (~375 miles). If you saw the Tour of California this year, one of the stages followed Highway 1 along the coast. The ride from Monterey to Santa Monica is unbelievably beautiful, exciting, challenging and very technical. It is a great drive via car but spectacular via bike. There are stretches of ocean views and winds, beautiful redwood forests, epic climbs and fast descents, switchbacks, no guard rail sections and sheer cliffs. We went with a paid camping tour that carried our equipment and provided meals and support. Traffic was not a problem when we did it, but depending on the time of year and week, it can be. As for hotels and B&B, there are many along the way, but we have no recommendations. We liked the ride from Monterey to Big Sur but it is all fantastic. This is advanced tandeming and requires a good level of fitness and technical skills. I am sure that there are bike tours that could provide assistance via B&Bs or hotels.
Napa and Sonoma are very easy to tandem cycle with a few technical areas. Crossing the ridge from Sonoma to Napa is a good climb but very doable with some curves and fast descents. The roads in Napa and Sonoma are generally smooth and good, may or may not have bike lanes and are usually rolling terrain with some climbing. Traffic is light during the week and heavier on the weekends. We stayed at the Geyserville Inn in Sonoma and rode various routes to wineries. One can do a similar thing in Napa. The Silverado Country Club (It is a private club but open to the public during certain times) has guest quarters and a 36 hole golf course such that you can stay there and cycle and golf. Silverado is first class accommodations and in the heart of Napa.
Whatever you can do, or dream you can, begin it. Boldness has genius, power and magic in it. - Johann Wolfgang von Goethe, writer