Hi, question: is it profitable for the same player to own two cities on the same island?
Also depends how many cities you have. Many players once they get to a certain size pretty much dispense with farming, maybe only farming a couple of times a day, or if you are lazy like me, once i get to about 20-30 cities i pretty much stop farming, so at this point the "farm" issue becomes a non-issue.
In red areas being able to back an offensive city up with on island defence is certainly a benefit.
Of course, come wonder phase, resources and farming do the become an issue so a lot of players with doubles on islands will swap cities out with other players to overcome that hurdle.