Two college professors own a principal residence and a second one. One of the profs is named president of his college for an interim period expected to last 2 years but which actually lasts approximately 4 years, during which a search is conducted to find a permanent president. During the 4 years or so, the profs are required by the college to reside full time at college expense in a house on campus, creatively called the president's residence. They left the other 2 residences unoccupied, save for the few weeks each year when they go to the second one for a vacation, and the occasional visit to the first one to check on it (never overnight). For a month or two, they allowed their son to live in the first residence. There was never any doubt that they would return to the first residence, and I think they continued to get some mail there. I'm pretty sure their drivers' licenses never changed addresses.
What is the FIT treatment of the 2 residences that they own during their residency in the president's residence? You know, the usual stuff, like deducting property tax and interest on both, and excluding gain, should they sell the first one. I think that section 119 is clearly applicable, but chime in on that too, if you wish. The college paid their rather minor moving costs, if that's important. If needed, the approximate move-out date was around 7-1-15 and the approximate move-back-in date was 6-30-19.