I forget the rule on such matters, but if this belongs in ASB, I certainly won't object.
Anyway, lets posit that our current theories on the formation of the Earth and Moon are correct; that the Earth, early in its history, was struck by a glancing blow from another planet, Theia, and that the aftermath of that impact came to form the Earth-Moon system, with this being the last major planetary body to collide with the Earth.
Instead, lets say that the two bodies instead form a binary system, with Theia taking the place of the Moon. Theia is about the size of Mars. Since much of that mass now comprises the Earth in our history, the alternate Earth would have less mass, and thus, less gravity. It might also be dryer, as much of the water delivered to the planet via asteroids and comets might instead land on Theia. On the other hand, if the total mass of the system is greater (I don't know if a significant portion of the combined mass was lost in the impact), then perhaps there'd be a greater number of impacts, balancing things out. Tidal forces on Earth would likely be much stronger as well.
Theia, on the other hand, would be large enough to support oceans and an atmosphere, much as ancient Mars did. Whether or not it would be able to maintain such conditions would seem to depend on Theia's magnetosphere. Perhaps, if it doesn't have one, Earth's magnetosphere would be sufficient to protect it.
Any other ideas people have on such a situation?