I had this very discussion the other day with a friend in Tx who is having hell with some Camp Cooley cattle that he bought who are throwing an unusually high number of twins.
In order for twinning to happen, a cow has two produce more than one viable egg, or the original egg splits. If multiple eggs are produced, you have fraternal twins (non-identical). If an egg splits you wind up with identical twins.
A herdsire, be it a bull, stud, buck or whatever will produce millions of sperm cells all fighting to be the first to reach the egg. Once a sperm cell fertilizes the egg, other or more sperm cells cant keep fertilizing to produce more offspring. One egg equals one offspring unless the egg splits during gestation. It is believed that twins are passed on through paternal rather than maternal genetics.
This man was having trouble with twins being born this years calf crop and was blaming it on the bulls that sired the calves. In actuality, his blame is misplaced and it is most likely the fault of the sire or sires of the cows. The bull determines the sex of the offspring, but the cow and number of eggs she produces, determines the number.
Keep in mind, I learned this over 20 years ago in college and I am sure that better research has been done since then.
Tex