So, how is it that Action Comics is reaching 1000 before Detective Comics?
Well, that takes time to tell...
May, 1939Superman shared the cover of Action Comics #12 with Zatara, the Magician, at the same time that Batman premiered in Detective Comics #27, with the difference in the issues being 15.
....while at the same time, on the cover of Detective Comics #152, Batman's girlfriend, Vicki Vale appeared, who was also a photographer at the time.
Neither of them can see Action Comics gaining on Detective Comics....as of yet.
There would be more of these extra issues with Supergirl in action!
So, now there are only 11 issues between the two series.
March, 1977Calculator) in Detective Comics #468 (while Bruce Wayne dealt with WGBS's Morgan Edge, having a little bit of a Superman crossover within this issue, though Superman was not in the issue).
Superman's Action would continue to gain as Batman's Detective remained bimonthly for a time, but there were more reasons for the larger gap....which we can go into at a later time.