Where did the idea of controlling 2 airports simultaneously come from? Several people seem concerned about that, and rightly so if it were to happen, which I'm sure will be on some office dweller's mind. At the moment though, the economies of scale for staffing in a single location rather than multiple isolated locations seems to be the driver, which also seems logical.
As for making it more difficult for an airport to change service provider, surely just feed data to a different location. A possible issue in conversion training if current provider is operating remote towers from single location as there are not necessarily any controllers with a vested life in the local area who may want to stay around, so a bit of a tougher negotiation on validation training pre or post handover, but otherwise can't see the issue.
Obviously, from a controller's perspective, it will doubtless end in a race to the bottom of the wages ladder, and the stereotypical "Bangalore call centre" option appearing on some bean counter's list...