It was my impression that you could get away with having a character with the same name as another company's (obviously, the Marvel and Fawcett/DC Captain Marvels have coexisted for decades). Where the complications lie in the usage is in the title of the comic book. Since Marvel grabbed the Captain Marvel title after this dreadful "split" version went belly up, DC couldn't use it when they decided to revive the old Fawcett character. That's why his book has always been called Shazam.