@_TheGeoff I checked. In the credits the sailing ship is still HMS and the biplane still has the Imperial German WW1 logo. The first obviously diverged image is the moon landing so presumably the split happened around WW2 or the early part of the Cold War.
@_TheGeoff In the Enterprise Mirror Universe episodes the opening credits show an aggressive human society going back to the first sailing ships. I think they diverged long before Cochrane shot the vulcan.