Sintered means the polyethylene (P-Tex is most common for snowboards) powder is slowly heated under high pressure and then cut to the desired shape. Double sintered simply means that after cooking the polyethylene once it is re-ground into a powder and the proccess is repeated. Where as extruded polyethylene is made by melting pellets and forcing the material through a nozzle of the desired shape. Sintered polyethylene is higher quality then extruded but, can be harder to repair. A sintered polyethelene has a "high molecular weight".
Thus comes the UHMW. UHMW simply stands for Ultra High Molecular Weight polyethylene. It's basically a high grade polyethylene that is incrediblely durable, has high abrasion resistance, a low coefficient of friction, and more impact resistance then any other polyethylene.
So the term Double Sintered UHMW or even Sintered UHMW is some what redundant because if it's sintered it means it has a high molecular weight. Company's could use the terms Double Sintered, Sintered, or UHMW by themselves and they would effectively mean the same thing. Using them together seems more like fancy lingo for marketing.