A coupling device (28) is formed of a double tubing (50) of a substantially non-porous membrane material, typically a conventional graft material, that is of inner and outer layers of membrane material (52, 54). The inner and outer layers (52, 54) are coupled by bridging rings (56, 58) which allow the layers (52, 54) to be spaced from one another in use. Attached to the inner and outer layers (52, 54) are first and second stents (60, 62). The stent (60) is located on the inside of the double tubing, while the stent (62) is located on the outside of the double tubing (50). The device (28) can expand in effect to ‘bulge’ and thus to fill the gaps to the vessel wall and to the stent-graft sections (24, 26). The device can provide reliable coupling of stent-grafts in vessels of varying diameter or in vessels inflicted with one or more aneurysms.