Having two parts or aspects; double.

• Designed for or involving two functions, users, or modes.
• Belonging to two at once, as in dual citizenship.
• In some languages, expressing the grammatical number two.

A counterpart related by duality, especially in mathematics.

• The dual space or dual object of a structure.
• A head-to-head meet between two teams.

To convert a single carriageway into a dual carriageway.