I agree with what you have said and it holds true in non-software related activities also.
For example, the effectiveness of a two person military unit compared to just a one person unit increases more than the expected 50%.
This is no magic formula, it is human nature - 2 brains are better than one.
In case of software writing, you have to decide at which point to apply the principle: during coding, after a code block is written, after it is checked in, etc
Secondly, you have to decide what you mean by ‘better code’: improve your skills, optimisation, functionality, etc
And adhere to this rule when finding a buddy: be the worst musician/guy in the band (from Chad Fowler who read it from guitarist, Pat Metheny)
Stijn.