According to flight crew statements, possible moderate turbulence was forecasted for portions of the route of flight and they briefed the flight attendants (FA) of the possible turbulence. During the first portion of the flight, several areas of turbulence were encountered. The seatbelt sign was turned on during this time and the captain told the FAs to suspend service and remain seated. The flight crew requested and was cleared to a lower altitude and the turbulence subsided. After the flight was in smooth air for several minutes, the captain advised the FAs they could leave their seats but not use the carts. About five minutes later the flight again encountered moderate to severe turbulence. The captain called the cabin and discovered two FAs in the aft galley had sustained injuries during the encounter but could continue their duties to the destination. After arrival, the FAs were transported to the hospital where one was diagnosed with a tibia plateau fracture.