There would be other occurrences and characters that would dramatically different had DC and Marvel been one company.
Do the X-men not occur, or does the Doom Patrol not happen, or are they both very different?
Carol Danvers doesn't exist as there is no Marvel to grab the name and the DC-vel universe has the original Captain Marvel all to themselves. Miracleman is still known as Marvelman, but is still decades late.
The FF and Spidey both exist. Yay!
Does Jean Loring become the Wasp?
Frank Rock, director of S.H.I.E.L.D.?
More personally, would Darkseid and the New Gods even exist if the one big superhero company meant no place for jack kirby to go to to get his chance to create his own way.
There are quite a few things we can speculate on and this is a far more interesting idea than I first thought.