None of the milestones in historical crossbow literature (Payne-Gallwey, R. : The Crossbow, Alm, J. : European Crossbows; Harmuth, E. Die Armbrust & Paterson, W.F. : A Guide to the Crossbow) mention or depict double crossbows, nor have I ever seen a non-modern one in my 20 years of bowyery (period crossbows and handbows).
There are extant examples of crossbows that shot two arrows at once, by means of two bolt grooves side by side but only one bow for propulsion.