The prefix "bi-" is commonly used in English to indicate "two" or "twice." It has multiple synonyms, each with a slightly different meaning. For example, "di-" is a synonym that also means "two" but often refers to chemical compounds. "Duo-" suggests a pair or team working together. "Twin-" refers specifically to two identical objects. "Dual-" suggests two distinct components that work together. "Bilateral-" and "diplo-" both suggest two sides or aspects of something. "Bin-" is a less common synonym that simply means "two," while "bis-" is often used to indicate something happening twice. The use of synonyms can help add variety and nuance to your writing.