Not altogether sure what is happening with TUI in 2020 at the moment as new flights are operating to Zante and extra flights to Corfu and Dalaman but according to my contact in TUI , flights to Ibiza and Mahon are not operating next year.
Considering both destinations have been operated from BOH since " whenever " it's quite a surprise . Queue Ryanair to IBZ again
Still leaves 3 gaps in the schedule at the moment, what could happen now is TUI reschedule flights and use some aircraft as W patterns from elsewhere which would then be no requirement for a 2nd aircraft
Watch this space though as still a year away