Exactly one developer should be responsible for a review. Otherwise, you get diffusion of responsibility, and both reviewers assume they can half-ass things. Also, even if they were diligent, you're needlessly creating work for yourself, and the marginal benefit of the second reviewer is very small. Having two reviewers is okay if you give them non-overlapping parts of the code to review.

You should definitely do code reviews. Their main benefit is in the long-term improvement in coding quality and coding ability of your team members.