I think Musk will keep them separate for now. This pure speculation, but I have the feeling that Tesla is a company Musk will feel comfortable relinquishing control over as it grows, while SpaceX is his baby.
Musk is very much motivated by progressing humanity. At this point, the electric car and solar revolutions are well underway (thanks in part to him). But if he were to die tomorrow, it's very possible that a lot of his dreams for space travel would die with him. The industry just hasn't reached the same level of competitive self-sufficiency.