A method and a plant for twin-sheet thermoforming of fuel tanks; first and second sheets (SA, SB) of thermoformable plastic material, are separately fed along respective processing lines (10A, 10B). The sheets (SA, SB) are heated and gripped along their peripheral edges by a pneumatically actuate suction and vacuum holding device (15A, 15B), for supporting the heated sheets (SA, SB) in a substantially flat condition while they are moved towards a respective thermoforming station (16A, 16B). Both the molds (17A, 17B) are disposed side by side with their open cavity facing upwards. After thermoforming of the plastic sheets (SA, SB), one of the molds (17B) is turned upside down onto the other mold (17A), to overlap and weld superimposed sealing areas of the two thermoformed shells (GA, GB). Cooling of the molds and thermoformed tank may be performed on a side of the processing lines (10A, 10B).