It's technically a violation of the conditions of carriage (the contract between you and the airline).

You should change the second leg of your original Mon - 2nd Fri ticket to the 1st Friday and purchase a new Mon - Fri ticket for the second week.

If you do it regularly to take advantage of the lower fares, and they catch you, they can bill you for the difference. Buying the tickets at the same time is a bad idea as it shows intent.

If you do it very rarely and for demonstratable reasons, it's unlikely to cause problems. For example, I had a Tue - 2nd Thu purchased 12 weeks out. Last minute I had to return for the middle weekend to take a sick animal to the vet, so I flew home Friday's red-eye, vet Sat AM, crashed, flew back Sun PM. Other than asking if I was OK, nobody said anything (and the out-station is a smaller one, I'm well known there).