[SpaceShouldBeMilitarized (OP)]

How different would our planet be if we had two moons?

[Pikaia]

That would depend how big they are and how far away. It could make the tides complicated!

[Soul Surfer]

The earth has got two moons!  after a fashion see http://www.helium.com/items/1789595-earths-second-moon-cruithne

this is a reasonable sized asteroid (3 miles across) in a stable synchronous orbit of one year with the earth.

[maffsolo]

How different would our planet be if we had two moons?

Two moons: positions of their orbits, size of the mass, the full orbital distance, orbit synchronization, if the moons have a revolutionary axes, and a mag field. These are some characteristics that can affect tides, night lighting, blue moon tracking, ancient calendars and events, tabulation of seasonal changes and historical event recording.

It just might affect the earths core temperature indirectly affecting the earth's magnetic field.

[Bill S]

Without wishing to second-guess SSBM, I wonder if the original question meant identical moons in orbits at similar distances. 

Wow! Imagine the possibility for collision. []