It does not mean double the cost if it takes less than 1/2 the time it would have taken with one dev. I think on difficult or low-level tasks this would be helpful. I find that it's worth it because you've got someone to say "no, don't do THAT!" long before it ends up in production code where it will REALLY cost you time and money.
I've written operating systems and things of that nature where it was invaluable that someone was sitting next to me to double check my logic.
