Re West coast, I've been revisiting my trip memories giving recommendations to some Aussies, and want to bring up these important topics.
-as mentioned, a two wheel trailer will bring lots of potential dangers as often the road can be narrow and you're on a tandem already. Don't forget, tourists and retired folks drive this route in cars and RVs so you gotta be on the ball. Some sections have logging trucks, or used to.

-it's not flat and there are many short challenging climbs, and a few long challenging climbs. So be aware of this and be realistic about your tandem climbing abilities and your fitness for climbs. My take on tandems is that they may be harder on hills? You certainly know better than I do...

It's not to be a downer , but just so you are aware.
Look at thread for the Aussies, USA trip suggestions, for more West coast comments.