What would happen if we have two moons?

quantanaut:

That really depends on a lot of different things, especially the mass of the second moon, and how far away it is. If Earth had a second moon, it would likely be smaller and further away than our current one is, which means it wouldn’t have a large effect on anything. If the second moon was instead larger and closer to our current one, it would likely be unstable. Because the two moons would interact so strongly, they would act like “slingshots” to each other. Either a moon would come too close to Earth and get torn apart, or a moon would get flung away.

This is something that might have happened on Mars a long time ago. It’s suspected that Mars had more than two moons in the past, but since this system was unstable, a moon got too close and was torn apart by Mars.