It's only against the CoC (contract of carriage) if the second ticket is used to circumvent the minimum stay requirements of the first ticket.

If neither ticket has minimum stay requirements, then there's no problem. There's also no problem if one uses two different carriers (and credits to different programs ) to circumvent minimum stay requirements.

However, obvious efforts to circumvent minimum stay requirements will eventually be noticed if one makes a habit of doing do. Penalties assessed by the airline could be a debit memo for the difference between a fare with and without minimum stay requirements and/or action taken against the FF account of the traveler.