From my experience though I often do make logical errors in my code but not in my tests and I do frequently catch errors because of this. I think thats a fairly normal experience with writing automated tests.

Would having someone else write the tests catch more logical errors? Very possibly, I haven't tried it but that sounds reasonable. It also does seem like that (and the other things it implies) would be a pretty extreme change in the speed of development. I can see it being worth it in some situations but honestly I don't see it as something practical for many types of projects.

What I don't understand is saying "well we can't do the really extremely hard version so let's not do the fairly easy version" which is how I took you original comment.