The term "bi-tumens" is not widely used in modern English vocabulary, and therefore, there are very few synonyms available for it. In fact, the term itself is a combination of two separate words, "bi" meaning two and "tumens" which is derived from the Latin word "tumere" meaning to swell or puff up. In essence, the term refers to a material that swells or expands when subjected to two different substances.
Therefore, some synonyms that could be used in place of "bi-tumens" depending on the intended meaning of the word could include "dual-reactive", "double-reactive", "dual-responsive", or "double-responsive". These words capture the notion of a material that reacts in two different ways depending on its environment.