I'm not sure if the Aero tank is the same, but earlier tanks had the oil compartment suspended inside the petrol tank - just the filler neck and the bottom fittings connecting the two together. The oil compartments also had cross tubes, so the tanks also acted as oil coolers while the petrol was circulating around them. The pipe across the bottom that connects the 2 sides of the fuel compartment is there to allow petrol to balance out across the 2 sides when the level gets below the height of the central tunnel.
Regards,
Eddie.