ChatterBank11 mins ago
2 moons
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For more on marking an answer as the "Best Answer", please visit our FAQ.The main effect of the Moon on the Earth is to produce tides (a high tide in the bit nearest the moon, another high tide on the other side of the world, and low tides in the bits in between. There are two tides per day (approximately) in any one place, because of the Earth rotating underneath where the Moon is, and the high tide follows the Moon around the Earth.
If there were 2 moons, each moon would have the same effect. If the 2 moons were at 90 degree angles to each other, the tides would cancel out with each other; if the 2 moons were at approximately the same angle (or opposite each other) then the tides which they produce would add together and make an extra big tide.
The cycle of tides coming in and out is an important part of the life of many creatures near the shore-line, so the existence of two moons would have made animals and plants evolve in a different way, dealing with more complex daily patterns of when and where the water is going to be.
http://en.wikipedia.org/wiki/Natural_satellite
Cruithne is the name of this "moon"