Most large companies have fairly strict rules that would prevent several key executives from traveling on the same plane.
Succession Planning is indeed an important part of a company's corporate strategy. The Board, usually working with VP of Human resources and the General Counsel, are generally responsible for this part of corporate governance. And I would be very surprised indeed if they didn't have plans in place in case any one of a dozen or more key Apple executives were to suddenly not be able to work.
Succession Planning, however, is very different from having an assumed or named "CEO-in-waiting" hanging around for an extended period. This is generally thought to be a very bad idea. For one thing, it tends to diminish the authority of the actual, present, CEO. People may think "why should I do what this guy says - instead I'll suck up to the guy who's going to be boss in the future.." In some cases it may even result in people sabotaging the efforts of the current CEO, hoping to hasten the day that regime change takes place.
The other reason having a "CEO-in-waiting" is a very bad idea is that it acts as a disincentive to other top managers. They may think to themselves "why should I keep knocking myself out, if I know I'm never going to be the top boss." It also makes those executives much more receptive to offers from other firms, who may wish to poach away top talent.
Obviously there are exceptions to this, an example being Apple's recent history, where the CEO had serious health issues necessitating lengthy leaves-of-absence and for someone to step in and assume the day-to-day responsibility for running the company. But this isn't the case with Apple now. Tim Cook has only been CEO for a few months. In fairness to him, and to the other top managers at Apple, it would be a very bad idea for anyone to become an "heir apparent."