The doubleneck tradition is "inherited" from his predecessor in the band, Ric Grech. You see another Gibson bass/guitar doubleneck here (with a very young - barely recognizable, but the voice gives him away - Chappo at a late '67 Family gig) at 3:30 (bass headstock) and 4:48 (full frontal view). But it's a different instrument, Wetton had the bass on top, Grech however the guitar.
Do not listen to this if, uhum, "more adventurous" horn harmonies are not your thing! I warned you.
Captain Beefheart fans will rejoice, however.
Of interest to our Dutch and Canadian contributors, Herr Grech also played other Gibson basses.
Wetton otoh played an EB-3 when he helped out Renaissance on a tour in 1971 (prior to Jon Camp becoming their steady bassist in 1972):
