I see this old post from Roger (previous owner of SLM) where he says they did switch from SS to Ronco in 1991 - and he does discuss using two 22 gal Ronco tanks connected together. If the connection was 1-1/2", maybe they just used a short nipple and mated the tanks together before installation. That seems frought with the danger of damage stressing the tank fittings during installation. I don’t think the interconnect needs to be 1-1/2" - that seems over kill. 1/2" or 3/4" would be good enough. The interconnect could then be by hose connecting 90 degree barbs mounted on opposing (port/sb) corners of tanks.