I believe they were twin Buick Nailhead V-8's driving through a single shaft.
In 1953, in the beautiful Harley Early-designed Skylark, Buick engineers introduced a unique V8 featuring a vertical Over Head Valve (OHV) head that featured a "pent-roof" shaped combustion chamber. This head design permits excellent low-end torque and fuel economy characteristics with a free-flowing (45-degree) intake tracks, but the design does compromise the exhaust ports' ability to breath at higher RPM's with restrictive 135-degree bends. It earned the nick-name "Nailhead" from the unique, vertical orientation of the valve covers, making it easy to identify Buick 264, 322, 364, 401, 425 CID OHV V8's prior to 1970. See Buick Engine specifications.