Ages ago, along with Angel Rodriguez of R+E, they figured out that stokers needed top tube length, too.
It was actually Glenn Erickson of R&E that pioneered the long stoker compartment... out of necessity, i.e., the mother of invention.
Despite the widely held belief that a longer stoker compartment would create handling issues, he was able to design a frameset that retained the stiffness needed for a performance tandem and then adopted it as his design standard while building frames at R&E. The tandems sold by R&E when Erickson & Rodriguez owned the business were branded as Rodriguez but they were designed by Erickson who eventually went on to establish and build tandems under his own brand about the same time that R&E was sold to new ownership. There is another recent post in the archives where I tried my best to recreate the time line and lineage.
I don't know if Bill Davidson used the longer stoker compartments as his default or merely built them to order that way when customers asked for a racing fit. My recollection is that most Davidsons don't use the very long stoker compartments that are commonly found on Rodriquez, Erickson and a few Bushnell tandems.
All of that said, Bob Freeman at Elliott Bay will probably know as much about that bike as anyone else short of Bill. The Davidson's are beautifully made-to-order tandems. A friend and his wife had one built as a wedding gift (I believe they registered at Elliott Bay) back in the mid-80's and spent their honeymoon riding it from Seattle, WA, to Redlands, CA. There are some incredible paint schemes on the Davidson's courtesy of a local painter. Probably not state of the art, but likely a work of art just the same.