1) Chang or Eng 2) Eng or Chang famously 
Siamese twin: Identical (monozygotic) twins that did not separate fully from one another but are still partially united. Due to the incomplete division of one fertilized ovum. Such twins are known medically as conjoined twins. Conjoined twins are popularly known as Siamese twins after Chang and Eng (1811-1874), the celebrated conjoined Chinese twin...
A rare phenomenon in which a pair of identical twins are born with their bodies joined at some point. Many conjoined twins are born stillborn because their particular condition was incompatible with life. Common joining parts include the head, buttocks, hips or back. Surgery to separate conjoined twins can be relatively simple or extremely com...
[n] - one of a pair of identical twins born with their bodies joined at some point
conjoined twin noun one of a pair of identical twins born with their bodies joined at some point