Can two different types of sperm fertilise one egg ?

Not normally, but...: In abnormal situations, stuff happens. In normal life, one type of sperm can fertilize one type of egg to make a successful offspring. It is species-specific, so only the human type of sperm fertilizes a human egg to successfully make a baby. If asking whether 2 human sperm can "fertilize" one egg, the answer is NO for getting a normal baby, but YES for getting a mole or an ultra-rare mosaic baby.