SoCal has gotten absolutely no love on this thread...it wouldn't be my first choice destination for a week of tandeming, but there is plenty of great cycling. I have not biked much in LA (with the exception of the river trails), so I'll focus my comments on Orange County.

The Bicycle Club of Irvine does a near-daily ride. http://www.bikeirvine.org
There are usually a few different lengths for riders of different abilities. I haven't been in Orange County in a while, but there used to be at least a half dozen tandems on the weekends.

After some falling out with the Bike Club of Irvine, the tandem club moved over to the OC Wheelmen. I'm not sure if the rides in their website are tandem exclusive...it used to be just one a month. http://www.ocwheelmen.org/page/show/...dem-time-rides

A fantastic ride runs from Newport Beach to San Diego...it is mostly flat to rolling (one real hill at La Jolla), and assuming there are no terror threats goes through Camp Pendleton. About 90 miles total--stop halfway if you want. Take the train back to Orange County (Amtrak is spotty with a tandem; take the local "Coaster" to Oceanside, then switch to OC Metro for a tandem friendly experience).