If the reason you need a 2 hr fire rated ceiling is due to an occupancy separation ie: dwelling below and storage above (usuable attic space) it is possible to obtain a two hr rating. You do need two layers of 5/8 type X sheetrock (standard 5/8 board won't do). The sheetrock must be on both sides of the structural members. On the ceiling side, the suppports must be a minimum of 16" OC- on the floor side, the sheetrock may be placed over the subfloor and covered with a solid flooring material. The floor side of the separation may also be covered with a ridgid cement board with the proper fire rating- or light weight concrete. Check with your local building offical. Be aware of the weight you're adding to the ceiling / floor structure- not only the sheetrock, but the additional weight from the added storage. What is the span of the 2X8 joists? Be sure not to overload your framing members.