"I respect people who can do both careers, like Will Smith and a couple of other people who have done it, but I just don't know when they sleep"

Val Kilmer is expressing adoration for those who can successfully handle 2 professions, such as Will Smith. He acknowledges that it is a challenging task to accomplish, and wonders how those who do it manage to discover the time to sleep. He is implying that it takes a great deal of devotion and hard work to be able to juggle two professions, which it is an impressive achievement. He is also suggesting that it is a difficult lifestyle to keep, as it needs a great deal of energy and time. Val Kilmer is expressing adoration for those who can handle to do both, and is also expressing his own disbelief at how they manage to do it. He is recommending that it is a challenging way of life to maintain, and that it needs a good deal of devotion and effort.