In touch up painting, do you have to worry about the border between the existing paint and the newly touched up area?
The touch up area probably won't be markedly different from the rest of the wall. It's already got 2 coats, it's the same can of paint and it's only been a few weeks. 

The difference between 1 coat and 2 coats of paint is usually fairly significant, but adding a third coat usually doesn't make a noticeable difference.