Image from Bernd Helfert
Biont clades which have been gengineered to consistently produce twin pregnancies are known as Twin Clades. While some of these clades produce dizygotic twins, most such clades produce monozygotic ('identical') twins. Monozygotic twins occur when an ovum splits in two post-fertilization, resulting in two genetically identical embryos.
Twins of this nature have long been noted for their great, though not total, similarities. While they are originally genetically identical, environmental factors, especially during gestation or early after birth or hatching, can produce epigenetic differences by turning different genes on or off. Epigenetic differences increase with age and are more pronounced when the twins have divergent experiences or upbringings. Nevertheless, monozygotic twins are often very similar in habits, tastes and personality, the latter of which has historically often been seen to become more similar with time.
Cryptophasia has long been observed in monozygotic twins, they often develop their own private languages in early life, especially if raised in somewhat non-verbal environments. Monozygotic twins are thought to have been studied since even before the Information Age on Old Earth to determine the importance of genetics in psychological development.
Among baselines, twinning of this nature was rare and essentially random with no genetic predispositions, but as early as the Interplanetary Age it began to become more common and some early clades started to develop, some for research purposes, others were founded by twins, often for the sake of continuing their lifestyle and creating a whole new culture. At first this was accomplished by manipulating in-vitro fertilization, but not too long after this time genetic studies of the Nine-Banded Armadillo (which always produces identical quadruplets) revealed a mechanism which could guarantee the production of identical twins in natural reproduction.
Since then, many Twin Clades have been created. There are clades made entirely of identical twins, others restrict the trait to one sex or another and often practice some form of bigamy. Some clades emphasize the sameness of the twins by encouraging twins to never be apart, sharing all the same experiences, wearing the same clothes, and keeping identical styles, others find interest in going the other direction to see how similar the twins end up without trying to, a cultural remnant of early scientific interest.
The fact that baseline monozygotic twins are genetically identical means that they have very similar thought patterns and they can sometimes seem almost telepathically linked, but this is just the result of them being very similar in genetics and experience, which can be compared in a limited fashion to recently diverged Copies. Some Twin Clades are noted for taking this phenomenon and making it a reality, giving twins some means of linking and/or sharing their minds to some degree. This ranges from simple enhancement of many empathic cues made unique to the twins, to DNI or genemods creating partial sharing up senses or even full techlepathy to the point of making them one person.
Source : http://www.orionsarm.com/eg-article/4f58da39e27c3