Not part of the code but good practice to use tied breakers. My frend of 30 years had his son working with an electrician, he was in the attic working on a multiwire circuit, and they had one of the breakers off. He came in contact with the neutral and they lost him. Since then I use tied breaker on all multiwire circuits. That back feed can get the best of us. The inspectors in this area ask for 2 pole on multiwire.
