The two shells to serve as both the recipient and sender of the sperm with a double breeding at the same time. This is to avoid the appearance that will occur from within the same breeding.
The two shells to act as both sender and recipient sperm for breeding pairs at the same time. This is to avoid the appearance inferior to occur from within the same breed.
Both the shell will serve as both the receiver and sender sperm to mate in the same time. Thus avoiding the recessive to arise from within the same breeding