General synonyms:
* Double
* Two-fold
* Binary
* Paired
* Twin
* Coupled
* Simultaneous
* Combined
* Multiple
More specific synonyms:
* Biphasic: (referring to two phases or stages)
* Dichotomous: (referring to a division into two contrasting parts)
* Bipolar: (referring to two opposite poles or extremes)
* Bilateral: (referring to two sides)
* Bifurcated: (referring to something that is divided into two branches)
* Ambiguous: (referring to something that has two possible meanings)
Other related terms:
* Double-edged
* Two-pronged
* Twofold
* Dualistic
The best synonym for "dual" will depend on the specific context in which it is being used.