Siamese twins (also irreversible binomials, binomials, binomial pairs, freezes) in the context of the English language refers to a pair or grouping of words that is used together as an idiomatic expression or collocation, usually conjoined by the words and or or. The order of elements cannot be reversed.

Redistribution and education subsidies are Siamese twins
Redistribution and education subsidies are siamese twins
Theoretical and analytical embryology of conjoined twins: part I: embryogenesis
Anatomic description of conjoined twins: a plea for standardized terminology.
Theoretical and analytical embryology of conjoined twins: Part II: Adjustments to union
Effect of molecular structure on mesomorphism. 12. Flexible-center Siamese-twin liquid crystalline diesters - a "prepolymer"model
[Conjoined twins--diagnostic problems, difficult decisions in choosing the most optimal management in one of the rarest pathologies ...
Conjoined twins: Theoretical embryologic basis
SecDF of Bacillus subtilis, a molecular Siamese twin required for the efficient secretion of proteins.
The embryology of conjoined twins