It seems there would be a fire hazard adapting dual 30 amp cords to a 50 amp cord that is plugged into a 50 amp marine receptacle unless there are 30 amp circuit breakers between the two cords. The conductors in the small 30 amp cords are not rated for the higher current. The reason for the large single cord is for greater amperage service. A single 50 amp cord rated at 250 volts is two 50 amp 120 volt conductors that are out of phase which gets us the 240 volt at 50 amps; both the 120 volt and 240 volt are 50 amp protected. The total capability at 120 volts is 100 amps or at 240 volts 50 amps. Dual 30 amp cords rated at 120 volts can be "in Phase" or "out of Phase" at the dock pedestal. For "in Phase" power there are simply two 30 amp power sources at 120 volts and no way to obtain 240 volts. The total capability is 120 volts at 60 amps. For "out of Phase" power there are two 30 amp power sources at 120 volts which across the two 120 volt cables is measured 240 volts (they are out of phase). The total capability at 120 volts is 60 amps or at 240 volt 30 amps. The bottom line is anything plugged into a circuit must be suitably rated for that circuit.