Dr. Fate and Dr. Doom in 1966--the could do this if DC and Marvel hadn't registered those names as trademarks. They could have done that either by actually registering them with the Patent and Trademark Office--in which case they names would probably appear on a cover with an R in a circle--or publishing them with their names featured on a cover with TM noted by the names. So when DC ran Dr. Fate and Hourman's teamup in Brave and Bold--they could have used that to establish trademark. If DC and Marvel had not established those trademarks, there was nothing to stop some other company from using the names. If it IS a trademark of another company, they could still use the character name in stories, but they would not be able to promote or market using that name (so Captain Marvel is Captain Marvel, but the comic books are Shazam, Power of Shazam, Shazam Family Archives, etc.--and the action figures, toys, Halloween costumes, Saturday morning show, etc., have to be Shazam).