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The birth of twins is not a normal process. In normal process of fertilization, male gamete fuses with female gamete and gives rise to a single diploid zygote. This zygote later on undergoes divisions and develops into an embryo. When this normal process undergoes some alterations then it results in the formation of twins. These altered conditions can be summarised as below:

1. First, where two eggs are fertilized by two separate sperms and give rise to two zygote. Both of these zygotes undergo implantation and develop into embryos. Such twins are called fraternal or dizygotic twins. The gender of these twins may be same or different.

2. Second, when one egg is fertilized by one sperm thereby giving rise to one zygote. This zygote is then split into two in the early gestational period. Such twins are the called identical or monozygotic twins. The gender of these twins is same.

No, 2 sperms can not fertilize the egg at the same time. During fertilization head of sperm penetrates the ovum.The head consists of a haploid nucleus and a cap-likeacrosome, which contains enzymes that aid in fertilisation.

The sperm induces changes in thezona pellucidalayer of the ovumand blocks the entry of other sperms. This ensures that only one sperm fertilises an ovum.