This is reflected in their typing, with Reshiram being Dragon/Dragon and Zekrom being Dragon/Electric.
Their lore also reinforces this:
* Reshiram: Represents truth, ideal, and light. It embodies the power of the "white" ideal, believing in truth and justice.
* Zekrom: Represents deceit, illusion, and darkness. It embodies the power of the "black" ideal, believing in power and might.
While both are dragon types, they are distinct in their symbolism and powers, with Reshiram representing light and Zekrom representing darkness.