It may take a while for new comers to productively bring this into practice, may be a few iterations. But once the team becomes used to it, the productivity gain may exceed the cost of having 2 persons working on the same code.
Code reviews can be greatly reduced or totally eliminated. Testing effort may be reduced as well as it is expected the pair- programmers have built a good piece of module or software.
Other side effects of practicing pair programming are each other can learn one or things from his/her partner. And guess what ?? If one programmer became unavailable due to any reason (has to be assigned to a higher priority project or leaves the company), the other person can still continue with almost no effect to project schedule !!