A programming practice (in)formally described and preached by eXtreme Programming methodology users.
Programmers work in pairs to create all production code. The idea is that the second programmer provides constant code review for the one who is "driving." If code review is good, then take it to the extreme by doing it all the time.
Ideally, two programmers working together can produce better quality code faster than two of them working alone. I guess this is synergy.
There are also other side benefits like constant mentoring and knowledge sharing. Pair programming also provides better communication in a development team.